Authorities.—A special treatise on the antiquities of Cephalonia was written by Petrus Maurocenus. See Holland’sTravels(1815); Ansted’sIonian Islands(1863); Viscount Kirkwall’sFour Years in Ionian Islands(1864); Wiebel’sDie Insel Kephalonia; parliamentary papers. Riemann,Recherches archéologiques sur les Iles Ioniennes(Paris, 1879-1880); Partsch,Kephallenia und Ithaka(1890); see alsoCorfu;Ionian Islands.
Authorities.—A special treatise on the antiquities of Cephalonia was written by Petrus Maurocenus. See Holland’sTravels(1815); Ansted’sIonian Islands(1863); Viscount Kirkwall’sFour Years in Ionian Islands(1864); Wiebel’sDie Insel Kephalonia; parliamentary papers. Riemann,Recherches archéologiques sur les Iles Ioniennes(Paris, 1879-1880); Partsch,Kephallenia und Ithaka(1890); see alsoCorfu;Ionian Islands.
(E. Gr.)
CEPHALOPODA,the fifth of the classes into which the zoological phylum Mollusca is divided (seeMollusca). The Cephalopoda are mainly characterized by the concrescence of the foot and head. The foot grows forward on each side so as to surround the mouth, the two upgrowths meeting on the dorsal side of the head—whence the name Cephalopoda. The perioral portion of the foot is drawn out into paired arm-like processes; these may be beset with sheathed tentacles or with suckers or hooks, or both. The epipodia are expanded into a pair of muscular lobes right and left, which are bent round towards one another so that their free margins meet and constitute a short tube—the siphon or funnel. The hind-foot is either very small or absent. A distinctive feature of the Cephalopoda is their bilateral symmetry and the absence of anything like the torsion of the visceral mass seen in the Anisopleurous Gastropoda.
The anus, although it may be a little displaced from the median line, is approximately median and posterior. The mantle-skirt is deeply produced posteriorly, forming a large sub-pallial chamber around the anus. By the side of the anus are placed the single or paired apertures of the nephridia, the genital apertures (paired only inNautilus, in female Octopoda, femaleOmmatostrephesand maleEledone), and the paired ctenidia. The visceral hump or dome is elevated, and may be very much elongated in a direction almost at right angles to the primary horizontal axis of the foot.A shell is frequently, but not invariably, secreted on the visceral hump and mantle-skirt. The shell is usually light in substance or lightened by air-chambers in correlation with the free-swimming habits of the Cephalopoda. It may be external or internal, that is, enclosed in folds of the mantle. Very numerous minute pigmented sacs, capable of expansion and contraction, and known as chromatophores, are usually present in the integument. The sexes are separate.The ctenidia are well developed as paired gill-plumes, serving as the efficient branchial organs (figs. 4, 24),The vascular system is very highly developed; the heart consists of a pair of auricles and a ventricle (figs. 12, 28). Branchial hearts are formed on the afferent vessels of the branchiae. It is not known to what extent the minute subdivision of the arteries extends, or whether there is a true capillary system.The pericardium is extended so as to form a very large sac, passing among the viscera dorsalwards and sometimes containing the ovary or testis—the viscero-pericardial sac—which opens to the exterior either directly or through the renal organs. It has no connexion with the vascular system. The renal organs are always paired sacs, the walls of which invest the branchial afferent vessels (figs. 28, 29). They open each by a pore into the viscero-pericardial sac, except inNautilus. The anal aperture is median and raised on a papilla. Jaws (fig. 6,e) and a radula (fig. 9) are well developed. The jaws have the form of powerful beaks, either horny or calcified (Nautilus), and are capable of inflicting severe wounds.Cerebral, pleural and pedal ganglia are present, but the connectives are shortened and the ganglia concentrated and fused in the cephalic region. Large special ganglia (optic, stellate and supra-buccal) are developed. Sense-organs are highly developed; the eye exhibits a very special elaboration of structure in the Dibranchiata, and a remarkable archaic form in the nautilus. Otocysts are present in all. The typical osphradium is not present, except inNautilus, but other organs are present in the cephalic region, to which an olfactory function is ascribed both inNautilusand in the other Cephalopoda.Hermaphroditism is unknown in Cephalopoda; male and female individuals always being differentiated. The genital aperture and duct is sometimes single, when it is the left; sometimes the typical pair is developed right and left of the anus. The males of nearly all Cephalopoda have been shown to be characterized by a peculiar modification of the arm-like processes or lobes of the fore-foot, connected with the copulative function. The term hectocotylization is applied to this modification (see figs. 6, 24). Elaborate spermatophores or sperm-ropes are formed by all Cephalopoda, and very usually the female possesses special capsule-forming and nidamental glands for providing envelopes to the eggs (fig. 4,g.n.). The egg is large, and the development is much modified by the presence of an excessive amount of food-material diffused in the protoplasm of the egg-cell. Trochosphere and veliger stages of development are consequently not recognizable.
The anus, although it may be a little displaced from the median line, is approximately median and posterior. The mantle-skirt is deeply produced posteriorly, forming a large sub-pallial chamber around the anus. By the side of the anus are placed the single or paired apertures of the nephridia, the genital apertures (paired only inNautilus, in female Octopoda, femaleOmmatostrephesand maleEledone), and the paired ctenidia. The visceral hump or dome is elevated, and may be very much elongated in a direction almost at right angles to the primary horizontal axis of the foot.
A shell is frequently, but not invariably, secreted on the visceral hump and mantle-skirt. The shell is usually light in substance or lightened by air-chambers in correlation with the free-swimming habits of the Cephalopoda. It may be external or internal, that is, enclosed in folds of the mantle. Very numerous minute pigmented sacs, capable of expansion and contraction, and known as chromatophores, are usually present in the integument. The sexes are separate.
The ctenidia are well developed as paired gill-plumes, serving as the efficient branchial organs (figs. 4, 24),
The vascular system is very highly developed; the heart consists of a pair of auricles and a ventricle (figs. 12, 28). Branchial hearts are formed on the afferent vessels of the branchiae. It is not known to what extent the minute subdivision of the arteries extends, or whether there is a true capillary system.
The pericardium is extended so as to form a very large sac, passing among the viscera dorsalwards and sometimes containing the ovary or testis—the viscero-pericardial sac—which opens to the exterior either directly or through the renal organs. It has no connexion with the vascular system. The renal organs are always paired sacs, the walls of which invest the branchial afferent vessels (figs. 28, 29). They open each by a pore into the viscero-pericardial sac, except inNautilus. The anal aperture is median and raised on a papilla. Jaws (fig. 6,e) and a radula (fig. 9) are well developed. The jaws have the form of powerful beaks, either horny or calcified (Nautilus), and are capable of inflicting severe wounds.
Cerebral, pleural and pedal ganglia are present, but the connectives are shortened and the ganglia concentrated and fused in the cephalic region. Large special ganglia (optic, stellate and supra-buccal) are developed. Sense-organs are highly developed; the eye exhibits a very special elaboration of structure in the Dibranchiata, and a remarkable archaic form in the nautilus. Otocysts are present in all. The typical osphradium is not present, except inNautilus, but other organs are present in the cephalic region, to which an olfactory function is ascribed both inNautilusand in the other Cephalopoda.
Hermaphroditism is unknown in Cephalopoda; male and female individuals always being differentiated. The genital aperture and duct is sometimes single, when it is the left; sometimes the typical pair is developed right and left of the anus. The males of nearly all Cephalopoda have been shown to be characterized by a peculiar modification of the arm-like processes or lobes of the fore-foot, connected with the copulative function. The term hectocotylization is applied to this modification (see figs. 6, 24). Elaborate spermatophores or sperm-ropes are formed by all Cephalopoda, and very usually the female possesses special capsule-forming and nidamental glands for providing envelopes to the eggs (fig. 4,g.n.). The egg is large, and the development is much modified by the presence of an excessive amount of food-material diffused in the protoplasm of the egg-cell. Trochosphere and veliger stages of development are consequently not recognizable.
The Cephalopoda are divisible into two orders, Tetrabranchiata and Dibranchiata, the names of which (due to Sir R. Owen) describe the number of gill-plumes present; but in fact there are several characters, of as great importance as those derived from the gills, by which the members of these two orders are separated from one another.
Order 1. Tetrabranchiata(= Schizosiphona, Tentaculifera).
Characters.—The inrolled lateral margins of the epipodia are not fused, but form a siphon by apposition (fig. 4). The circumoral lobes of the fore-foot carry numerous retractile tentacles, not suckers (fig. 6). There are two pairs of ctenidial gills (hence Tetrabranchiata), and two pairs of renal organs, consequently four renal apertures (fig. 4). The viscero-pericardial chamber opens by two independent apertures to the exterior, and not into the renal sacs. There are two oviducts (right and left) in the female, and two sperm-ducts in the male, the left duct in both sexes being rudimentary. A large external shell, either coiled or straight, is present, and is not enclosed by reflections of themantle-skirt. The shell consists of a series of chambers, the last-formed of which is occupied by the body of the animal, the hinder ones (successively deserted) containing gas (fig. 1). The pair of cephalic eyes are hollow chambers (fig. 14. A), opening to the exterior by minute orifices (pin-hole camera), and devoid of refractive structures. A pair of osphradia are present at the base of the gills (fig. 4,olf). Salivary glands are wanting. An ink-sac isnotpresent. Branchial hearts arenotdeveloped on the branchial afferent vessels.
a, Visceral hump.
b, Portion of the free edge of the mantle-skirt reflected on to the shell,—the edge of the mantle-skirt can be traced downwards and forwards around the base of the mid-foot or siphon i.
l, l, Superficial origin of the retractor muscle of the mid-foot (siphon), more or less firmly attached to the shell, of which a small piece (s) is seen between the lettersl, l.
s, (farther back) points to the siphuncular pedicle, which is broken off short and not continued, as in the perfect state, through the whole length of the siphuncle of the shell, also marked s and s’.
o, points to the right eye.
t, is placed near the extremities of the contracted tentacles of the outer or annular lobe of the fore-foot—the jointed tentacles are seen protruding a little from their long cylindrical sheaths.
v, The dorsal “hood” formed by an enlargement in this region of the annular lobe of the fore-foot (min figs. 2, 3).
V, A swelling of the mantle-skirt, indicating the position on its inner face of the nidamental gland (see fig. 4,g.n.).
Visceral Hump and Shell.—The visceral hump ofNautilus(if we exclude from consideration the fine siphuncular pedicle which it trails, as it were, behind it) is very little, if at all, affected by the coiled form of the shell which it carries, since the animal always slips forward in the shell as it grows, and inhabits a chamber which is practically cylindrical (fig. 1). Were the deserted chambers thrown off instead of being accumulated behind the inhabited chamber as a coiled series of air-chambers, we should have a more correct indication in the shell of the extent and form of the animal’s body. Amongst Gastropods it is not very unusual to find the animal slipping forward in its shell as growth advances and leaving an unoccupied chamber in the apex of the shell. This may indeed become shut off from the occupied cavity by a transverse septum, and a series of such septa may be formed, but in no Gastropod are these apical chambers known to contain a gas during the life of the animal in whose shell they occur. A further peculiarity of the nautilus shell and of that of the allied extinctAmmonites,Scaphites,Orthoceras, &c., and of the livingSpirula, is that the series of deserted air-chambers is traversed by a cord-like pedicle extending from the centro-dorsal area of the visceral hump to the smallest and first-formed chamber of the series. No structure comparable to this siphuncular pedicle is known in any other Mollusca. The siphuncle does not communicate with the coelomic cavity; it is a simple vascular process of the mantle, whose cavity consists of a venous sinus, and whose wall contains a ramification of the pallial artery. There appears to be no doubt that the deserted chambers of the nautilus shell contain in the healthy living animal a gas which serves to lessen the specific gravity of the whole organism. This gas is said to be of the same composition as the atmosphere, with a larger proportion of nitrogen. With regard to its origin we have only conjectures. Each septum shutting off an air-containing chamber is formed during a period of quiescence, probably after the reproductive act, when the visceral mass of the nautilus may be slightly shrunk, and gas is secreted from the dorsal integument so as to fill up the space previously occupied by the animal. A certain stage is reached in the growth of the animal when no new chambers are formed. The whole process of the loosening of the animal in its chamber and of its slipping forward when a new septum is formed, as well as the mode in which the air-chambers may be used as a hydrostatic apparatus, and the relation to this use, if any, of the siphuncular pedicle, is involved in obscurity, and is the subject of much ingenious speculation. In connexion with the secretion of gas by the animal, besides the parallel cases ranging from the protozoonArcellato the physoclistic fishes, from the hydroidSiphonophorato the insect-larvaCorethra, we have the identical phenomenon observed in the closely alliedSepiawhen recently hatched. Here, in the pores of the internal rudimentary shell, gas is observable, which has necessarily been liberated by the tissues which secrete the shell, and not derived from any external source (Huxley).
The coiled shell ofNautilus, and of the majority of extinct Tetrabranchiata, is peculiar in its relation to the body of the animal, inasmuch as the curvature of the coil proceeding from the centro-dorsal area is towards the head or forwards, instead of away from the head and backwards as in other discoid coiled shells such asPlanorbis; the coil is in fact absolutely reversed in the two cases. Such a shell is said to be exogastric. But in some extinct forms,e.g.Phragmoceras,Cyrtoceras,Ptenoceras, the shell is coiled towards the ventral side, when it is termed endogastric. Amongst the extinct allies of the nautilus (Tetrabranchiata) we find shells of a variety of shapes, open coils such asScaphites, leading on to perfectly cylindrical shells with chamber succeeding chamber in a straight line (Orthoceras), whence again we may pass to the corkscrew spires formed by the shell ofTurrilites. In some extinct genera,e.g.Gomphoceras, among the Nautiloidea the aperture of the shell is contracted and the edge of the aperture is lobed. In these cases the animal was probably able only to protrude its appendages and not its whole head. The ventral part of the aperture corresponding to the funnel is separated from the dorsal part by a constriction. Hence it is possible to distinguish the ventral and dorsal sides of the shell and to decide whether it was exogastric or endogastric. The direction of the coil of the shell cannot be determined by the position of the siphuncle, which traverses the septa centrally, ventrally or dorsally. Contracted shell apertures occur also in Ammonitoidea, the condition reaching an extreme inMorphoceras, where the original aperture is subdivided by the ingrowth of the sides, so that only five small separate apertures remain. Of these the central probably corresponded to the mouth, two lateral to the eyes, and the remaining two to the pedal appendages.
Head, Foot, Mantle-skirt and Sub-pallial Chamber.—In the pearly nautilus the ovoid visceral hump is completely encircled by the free flap of integument known as mantle-skirt (figs. 2, 3,d, e). In the antero-dorsal region this flap is enlarged so as to be reflected a little over the coil of the shell which rests on it. In the postero-ventral region the flap is deepest, forming an extensive sub-pallial chamber, at the entrance of which e is placed in fig. 3. A view of the interiorof the sub-pallial chamber, as seen when the mantle-skirt is retroverted and the observer faces in the direction indicated by the reference line passing from e in fig. 3, is given in fig. 4. With this should be compared the similar view of the sub-pallial chamber of the DibranchiateSepia. It should be noted as a difference betweenNautilusand the Dibranchiates that in the former the nidamental gland (in the female) lies on that surface of the pallial chamber formed by the dependent mantle-flap (fig. 4,g.n.; fig. 1, V), whilst in the latter it lies on the surface formed by the body-wall; in fact in the former the base of the fold forming the mantle-skirt comprises in its area a part of what is unreflected visceral hump in the latter.Fig.2.—Spirit specimen of female Pearly Nautilus, removed from its shell, and seen from the antero-dorsal aspect (drawn from nature by A.G. Bourne).m, The dorsal “hood” formed by the enlargement of the outer or annular lobe of the fore-foot, and corresponding to the sheaths of two tentacles (g, gin fig. 6).n, Tentacular sheaths of lateral portion of the annular lobe.u, The left eye.b, The nuchal plate, continuous at its right and left posterior angles with the root of the mid-foot, and corresponding to the nuchal cartilage of Sepia.c, Visceral hump.d, The free margin of the mantle-skirt, the middle letterdpoints to that portion of the mantle-skirt which is reflected over a part of the shell as seen in fig. 1,b; the cup-like fossa to which b and d point in the present figure is occupied by the coil of the shell.g.a.points to the lateral continuation of the nuchal plate b to join the root of the mid-foot or siphon.Fig.3.—Lateral view of the same specimen as that drawn in fig. 2. Letters as in that figure with the following additions—e, points to the concave margin of the mantle-skirt leading into the sub-pallial chamber.g, The mid-foot or siphon.k, The superficial origin of its retractor muscles closely applied to the shell and serving to hold the animal in its place.l, The siphuncular pedicle of the visceral hump broken off short.v, v, The superior and inferior ophthalmic tentacles.The apertures of the two pairs of renal sacs, of the viscero-pericardial sac, of the genital ducts, and of the anus, are shown in position on the body-wall of the pallial chamber ofNautilusin figs. 4, 5. There are nine apertures in all, one median (the anus) and four paired. Besides these apertures we noticetwopairs of gill-plumes which are undoubtedly typical ctenidia, and a short papilla (the osphradium) between each anterior and posterior gill-plume (see figs. 4, 5, and explanation). As compared with this in a Dibranchiate, we find (fig. 25) only four apertures, viz. the median anus with adjacent orifice of the ink-sac, the single pair of renal apertures, and one asymmetrical genital aperture (on the left side) except in female Octopoda and a few others, where the genital ducts and their apertures are paired. No viscero-pericardial pores are present on the surface of the pallial chamber, since in the Dibranchiata the viscero-pericardial sac opens by a pore into each nephridium instead of directly to the surface. A single pair of ctenidia (gill-plumes) is present instead of the two pairs inNautilus. The existence of two pairs of ctenidia and of two pairs of renal sacs inNautilus, placed one behind the other, is highly remarkable. The interest of this arrangement is in relation to the general morphology of the Mollusca, for it is impossible to view this repetition of organs in a linear series as anything else than an instance of metameric segmentation, comparable to the segmentation of the ringed worms and Arthropods. The only other example which we have of this metamerism in the Mollusca is presented by the Chitons. There we find not two pairs of ctenidia merely, but sixteen pairs (in some species more) accompanied by a similar metamerism of the dorsal integument, which carries eight shells. InChitonthe renal organs are not affected by the metamerism as they are inNautilus. It is impossible on the present occasion to discuss in the way which their importance demands the significance of these two instances among Mollusca of incomplete or partial metamerism; but it would be wrong to pass them by without insisting upon the great importance which the occurrence of these isolated instances of metameric segmentation in a group of otherwise unsegmented organisms possesses, and the light which they may be made to throw upon the nature of metameric segmentation in general.Fig.4.—View of the postero-ventral surface of a female Pearly Nautilus, the mantle-skirt (c) being completely reflected so as to show the inner wall of the sub-pallial chamber (drawn from nature by A.G. Bourne).a, Muscular band passing from the mid-foot to the integument.b, The valve on the surface of the funnel, partially concealed by the inrolled lateral margin of the latter.c, The mantle-skirt retroverted.an, The median anus.x, Post-anal papilla of unknown significance.g.n., Nidamental gland.r.ov, Aperture of the right oviduct.l.ov, Aperture of the rudimentary left oviduct (pyriform sac of Owen).neph.a, Aperture of the left anterior renal sac.neph.p, Aperture of the left posterior renal sac.viscper, Left aperture of the viscero-pericardial sac.olf, The left osphradium placed near the base of the anterior gill-plume.The four gill-plumes (ctenidia) are not lettered.The foot and head ofNautilusare in the adult inextricably grown together, the eye being the only part belonging primarily to the head which projects from the all-embracing foot. The fore-foot or front portion of the foot has the form of a number of lobes carrying tentacles and completely surrounding the mouth (figs. 2, 3). The epipodia incline towards each other posteriorly so as to form an incomplete siphon (fig. 4), a condition which is completed and rendered permanent in the tubular funnel of Dibranchiata. The epipodial nature of the funnel is well seen in young embryos, in which this organ is situated laterally and posteriorly between the mantle and the foot.Fig.5.—View of the postero-ventral surface of a male Pearly Nautilus, the mantle-skirt (c) being completely reflected so as to show the inner wall of the sub-pallial chamber, and the four ctenidia and the foot cut short (drawn from nature by A.G. Bourne).pe, Penis, being the enlarged termination of the right spermatic duct;l.sp, aperture of the rudimentary left spermatic duct (pyriform sac of Owen). Other letters as in fig. 4.The lobes of the fore-foot ofNautilusand of the other Cephalopoda require further description. It has been doubted whether these lobes were rightly referred (by T.H. Huxley) to the fore-foot, and it has been maintained by some zoologists (H. Grenadier, H. vonJhering) that they are truly processes of the head. It appears to be impossible to doubt that the lobes in question are the fore-portion of the foot, when their development is examined (see fig. 35), further, when the fact is considered that they are innervated by the pedal ganglion. The fore-foot ofNautiluscompletely surrounds the buccal cone (fig. 6, e), so as to present an appearance with its expanded tentacles similar to that of the disk of a sea-anemone (Actinia). A.G. Bourne, of University College, prepared from actual specimens the drawings of this part in the male and femaleNautilusreproduced in fig. 6, and restored the parts to their natural form when expanded. The drawings show very strikingly the difference between male and female. In the females (lower figure), we observe in the centre of the disk the buccal cone e carrying the beak-like pair of jaws which project from the finely papillate buccal membrane. Three tentaculiferous lobes of the fore-foot are in immediate contact with this buccal cone; they are the right and left (c, c) inner lobes, and the inferior inner lobe (d)—called inferior because it really lies ventralwards of the mouth. This inner inferior lobe is clearly a double one, representing a right and left inner inferior lobe fused into one. A lamellated organ on its surface, known as Owen’s organ, probably olfactory in function (n), marks the separation of the constituent halves of this double lobe. Each half carries a group of fourteen tentacles. The right and the left inner lobes (c, c) each carry twelve tentacles. External to these three lobes the muscular substance of the mouth-embracing foot is raised into a wide ring, which becomes especially thick and large in the dorsal region where it is notably modified in form, offering a concavity into which the coil of the shell is received, and furnishing a protective roof to the retracted mass of tentacles. This part of the external annular lobe of the fore-foot is called the “hood” (figs. 2, 3,m). The median antero-posterior line traversing this hood exactly corresponds to the line of concrescence of the two halves of the fore-foot, which primitively grew forward one on each side of the head, and finally fused together along this line in front of the mouth. The tentacles carried by the great annular lobe are nineteen on each side, thirty-eight in all. They are called “digital,” and are somewhat larger than the “labial” tentacles carried on the three inner lobes. The dorsalmost pair of tentacles (markedgin fig. 6) are the only ones which actually belong to that part of the disk which forms the great dorsal hood m. The hood is, in fact, to a large extent formed by the enlarged sheaths of these two tentacles. All the tentacles of the circumoral disk are set in remarkable tubular sheaths, into which they can be drawn. The sheaths of some of those belonging to the external or annular lobe are seen in fig. 3, marked n. The sheaths are muscular as well as the tentacles, and are simply tubes from the base of which the solid tentacle grows. The functional significance of this sheathing arrangement is as obscure as its morphological origin. With reference to the latter, it appears highly probable that the tubular sheath represents the cup of a sucker such as is found on the fore-foot of the Dibranchiata. In any case, it seems to the writer impossible to doubt that each tentacle, and its sheath on a lobe of the circumoral disk of Nautilus, corresponds to a sucker on such a lobe of a Dibranchiate. W. Keferstein follows Sir R. Owen in strongly opposing this identification, and in regarding such tentacle as the equivalent of a whole lobe or arm of a Decapod or Octopod Dibranch. The details of these structures, especially in the facts concerning the hectocotylus and spadix, afford the most conclusive reasons for dissenting from Owen’s view. On the ventral side an extensive part of the internal surface of the muscular ring is laminated, forming the so-called “organ of Valenciennes,” peculiar to the female and serving for the attachment of the spermatophores. We have so far enumerated in the female nautilus ninety tentacles. Four more remain which have a very peculiar position, and almost lead to the suggestion that the eye itself is a modified tentacle. These remaining tentacles are placed one above (before) and one below (behind) each eye, and bring up the total to ninety-four (fig. 3v, v).Fig.6.—Male (upper) and female (lower) specimens ofNautilus pompiliusas seen in the expanded condition, the observer looking down on to the buccal conee; one-third the natural size linear. The drawings have been made from actual specimens by A.G. Bourne, B. Sc., University College, London.a, The shell.b, Theouterring-like expansion (annular lobe) of the circumoral muscular mass of the fore-foot, carrying nineteen tentacles on each side—posteriorly this is enlarged to form the “hood” (markedvin fig. 1 andmin figs. 2 and 3). giving off the pair of tentacles markedgin the present figure.c, The right and left inner lobes of the fore-foot, each carrying twelve tentacles in the female, in the male subdivided intop, the “spadix” or hectocotylus on the left side, andq, the “anti-spadix,” a group of four tentacles on the right side—it is thus seen that the subdivided right and left inner lobes of the male correspond to the undivided right and left inner lobes of the female.d, The inner inferior lobe of the fore-foot, a bilateral structure in the female carrying two groups, each of fourteen tentacles, separated from one another by a lamellated organn, supposed to be olfactory in function—in the male the inner inferior lobe of the fore-foot is very much reduced, and has the form of a paired group of lamellae (din the upper figure).e, The buccal cone, rising from the centre of the three inner lobes, and fringing the protruded calcareous beaks or jaws with a series of minute papillae.f, The tentacles of the outer circumoral lobe or annular lobe of the fore-foot projecting from their sheaths.g, The two most posterior tentacles of this series belonging to that part of the annular lobe which forms the hood (min figs. 2 and 3).i, Superior ophthalmic tentacle.k, Inferior ophthalmic tentacle.l, Eye.m, Paired laminated organ on each side of the base of the inner inferior lobe (d) of the female.n, Olfactory lamellae upon the inner inferior lobe (in the female).o, The siphon (mid-foot).p, The spadix (in the male), the hectocotylized portion of the left inner lobe of the fore-foot representing four modified tentacles, eight being left unmodified.q, The anti-spadix (in the male), being four of the twelve tentacles of the right inner lobe of the fore-foot isolated from the remaining eight, and representing on the right side the differentiated spadix of the left side. The four tentacles of the anti-spadix are set, three on one base and one on a separate base.In the adult male nautilus we find the following important differences in the tentaculiferous disk as compared with the female (see upper drawing in fig. 6). The inner inferior lobe is rudimentary, and carries no tentacles. It is represented by three groups of lamellae (d), which are not fully exposed in the drawing. The right and left inner lobes are subdivided each into two portions. The right shows a larger portion carrying eight tentacles, and smaller detached groups (q) of four tentacles, of which three have their sheaths united whilst one stands alone. These four tentacles may be called the “anti-spadix.” The left inner lobe shows a similar larger portion carrying eight tentacles, and a curious conical body behind it corresponding to the anti-spadix. This is the “spadix.” It carries no tentacles, but is terminated by imbricated lamellae. These lamellae appear to represent the four tentacles of the anti-spadix of the right internal lobe, and are generally regarded as corresponding to that modification of the sucker-bearing arms of male Dibranchiate Siphonopods to which the name “hectocotylus” is applied. The spadix is in fact the hectocotylized portion of the fore-foot of the male nautilus. The hectocotylized arm or lobe of male Dibranchiata is connected with the process of copulation, and in the male nautilus the spadix has probably a similar significance, though it is not possible to suggest how it acts in this relation. It is important to observe that the modification of the fore-foot in the male as compared with the female nautilus is not confined to the existence of the spadix. The anti-spadix and the reduction of the inner inferior lobe are also male peculiarities. The external annular lobe in the male does not differ from that of the female; it carries nineteen tentacles on each side. The four ophthalmic tentacles are also present. Thus in the male nautilus we find altogether sixty-two tentacles, the thirty-two additional tentacles of the female being represented by lamelliform structures.Musculature, Fins and, Cartilaginous Skeleton.—Without entering into a detailed account of the musculature ofNautilus, we may point out that the great muscular masses of the fore-foot and of the mid-foot (siphon) are ultimately traceable to a large transverse mass of muscular tissue, the ends of which are visible through the integument on the right and left surfaces of the body dorsal of the free flap of the mantle-skirt (fig. 1,l,l, and fig. 3,k). These muscular areae have a certain adhesion to the shell, and serve both to hold the animal in its shell and as the fixed supports for the various movements of the tentaculiferous lobes and the siphon. They are to be identified with the ring-like area of adhesion by which the foot-muscle of the limpet is attached to the shell of that animal. In the Dibranchs a similar origin of the muscular masses of the fore-foot and mid-foot from the sides of the shell—modified, as this is, in position and relations—can be traced.Fig.7.—Minute structure of the cartilage ofLoligo(from Gegenbaur, after Furbringer)a, Simple cells.b, Dividing cells.c, Canaliculi.d, An empty cartilage capsule with its pores.e, Canaliculi in section.InNautilusthere are no fin-like expansions of the integument, whereas such occur in the Decapod Dibranchs along the sides of the visceral hump (figs. 15, 16). As an exception among Octopoda lateral fins occur inPinnoctopus(fig. 38, A), and inCirrhoteuthis(fig. 38, D).InNautilusthere is a curious plate-like expansion of integument in the mid-dorsal region just behind the hood, lying between that structure and the portion of mantle-skirt which is reflected over the shell. This is shown in fig. 2,b. If we trace out the margin of this plate we find that it becomes continuous on each side with the sides of the funnel. InSepiaand other Decapods (not in Octopods) a closely similar plate exists in an exactly corresponding position (seebin figs. 10, 26). InSepiaa cartilaginous development occurs here immediately below the integument forming the so-called “nuchal plate,” drawn in fig. 8, D. The morphological significance of this nuchal lamella, as seen both inNautilusand inSepia, is not obvious. Cartilage having the structure shown in fig. 7 occurs in various regions of the body of Cephalopoda. In all Glossophorous Mollusca the lingual apparatus is supported by internal skeletal pieces, having the character of cartilage; but in the Cephalopoda such cartilage has a wider range.InNautilusa large H-shaped piece of cartilage is found, forming the axis of the funnel (fig. 8, A, B). Its hinder part extends up into the head and supports the peri-oesophageal nerve-mass (a), whilst its two anterior rami extend into the tongue-like siphon. InSepia, and Dibranchs generally, the cartilage takes a different form, as shown in fig. 8, C. The processes of this cartilage cannot be identified in any way with those of the capito-pedal cartilage ofNautilus. The lower larger portion of this cartilage inSepiais called the cephalic cartilage, and forms a complete ring round the oesophagus; it completely invests also the ganglionic nerve-collar, so that all the nerves from the latter have to pass through foramina in the cartilage. The outer angles of this cartilage spread out on each side so as to form a cup-like receptacle for the eyes. The two processes springing right and left from this large cartilage in the median line (fig. 8, C) are the “pre-orbital cartilages”; in front of these, again, there is seen a piece like an inverted T, which forms a support to the base of the “arms” of the fore-foot, and is the “basi-brachial” cartilage. The Decapod Dibranchs have, further, the “nuchal cartilage” already mentioned, and inSepia, a thin plate-like “sub-ostracal” or (so-called) dorsal cartilage, the anterior end of which rests on and fits into the concave nuchal cartilage. In Octopoda there is no nuchal cartilage, but two band-like “dorsal cartilages.” In Decapods there are also two cartilaginous sockets on the sides of the funnel—“siphon-hinge cartilages”—into which fleshy knobs of the mantle-skirt are loosely fitted. InSepia, along the whole base-line of each lateral fin of the mantle (fig. 15), is a “basi-pterygial cartilage.” It is worthy of remark that we have, thus developed, in Dibranch Cephalopods a more complete internal cartilaginous skeleton than is to be found in some of the lower vertebrates. There are other instances of cartilaginous endo-skeleton in groups other than the Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms a skeletal support for the gill-plumes, whilst in the Arachnids (Mygale,Scorpio) and inLimulusa large internal cartilaginous plate—the ento-sternite—is developed as a support for a large series of muscles.Fig.8.—Cartilaginous skeleton of Cephalopoda (after Keferstein.)A, Capito-pedal cartilage ofNautilus pompilius.apoints to the ridge which supports the pedal portion of the nerve-centre.B, Lateral view of the same—the large anterior processes are sunk in the muscular substance of the siphon.C, Cephalic cartilages ofSepia officinalis.D, Nuchal cartilage ofSepia officinalis.Alimentary Tract.—The buccal cone ofNautilusis terminated by a villous margin (buccal membrane), surrounding the pair of beak-like jaws, of which the ventral projects over the dorsal. These are very strong and dense inNautilus, being calcified. Fossilized beaks of Tetrabranchiata are known under the name of rhyncholites. In Dibranchs the beaks are horny, but similar in shape to those ofNautilus. They resemble in general those of a parrot, the lower beak being the larger and overlapping the upper or dorsal beak. The lingual ribbon and odontophoral apparatus have the structure which is typical for Glossophorous Mollusca. In fig. 9, A is represented a single row of teeth from the lingual ribbon ofNautilus, and in fig. 9, B, C, of other Cephalopoda.InNautilusa long and wide crop or dilated oesophagus (fig. 10,cr) passes from the muscular buccal mass, and at the apex of the visceral hump passes into a highly muscular stomach, resembling the gizzard of a bird (fig 10,gizz). A nearly straight intestine passes from the muscular stomach to the anus, near which it develops a small caecum. In other Cephalopods the oesophagus is usually narrower and the muscular stomach more capacious, whilst a very important feature in the alimentary tract is formed by the caecum. In all butNautilusthe caecum lies near the stomach, and may be very capacious—much larger than the stomach inLoligo vulgaris—or elongated into a spiral coil. The simple U-shaped flexure of the alimentary tract, as seen in fig. 10, is the only important one which it exhibits in the Cephalopoda. The acini of the large liver ofNautilusare compacted into a solid reddish-brown mass by a firmmembrane, as also is the case in the Dibranchiata. The liver has four paired lobes inNautilus, which open by two bile-ducts into the alimentary canal at the commencement of the intestine. The bile-ducts unite before entering the intestine. In Dibranchiata the two large lobes of the liver are placed antero-dorsally (beneath the shell in Decapoda), and the bile-ducts open into the caecum. Upon the bile-ducts in Dibranchiata are developed yellowish glandular diverticula, which are known as “pancreas,” though neither physiologically nor morphologically is there any ground for considering either the so-called liver or the so-called pancreas as strictly equivalent to the glands so denominated in the Vertebrata. InNautilusthe equivalents of the pancreatic diverticula of the Dibranchs can be traced upon the relatively shorter bile-ducts.Fig.9.—Lingual dentition of Cephalopoda. A, A single row of lingual teeth ofNautilus pompilius(after Keferstein). B, Two rows of lingual teeth ofSepia officinalis(after Troschel). C, Lingual teeth ofEledone cirrhosa(after Loven).Fig.10.—Diagram representing a vertical approximately median antero-posterior section ofNautilus pompilius(from a drawing by A.G. Bourne). The parts which are quite black are the cut muscular surfaces of the foot and buccal mass.a, The shell.b, The nuchal plate, identical with the nuchal cartilage ofSepia(see fig. 2,b).c, The integument covering the visceral hump.d, The mantle flap or skirt in the dorsal region where it rests against the coil of the shell.e, The inferior margin of the mantle-skirt resting on the lip of the shell represented by the dotted line.f, The pallial chamber with two of the four gills.g, The vertically cut median portion of the mid-foot (siphon).h, The capito-pedal cartilage (see fig. 8).i, The valve of the siphon.l, The siphuncular pedicle (cut short).m, The hood or dorsal enlargement of the annular lobe of the fore-foot.n, Tentacles of the annular lobe.p, Tentacles of inner inferior lobe.q, Buccal membrane.r, Upper jaw or beak.s, Lower jaw or beak.t, Lingual ribbon.x, The viscero-pericardial sac.n.c, Nerve-collar.oe, Oesophagus.cr, Crop.gizz, Gizzard.int, Intestine.an, Anus.nept, Aperture of a nephridial sac.r.e, Renal glandular masses on the walls of the afferent branchial veins (see fig. 11).a.b.v, Afferent branchial vessel.e.b.v, Efferent branchial vessel.vt, Ventricle of the heart.Posterior salivary glands are not developed inNautilus, but on each side in the wall of the buccal mass is a gland corresponding to the anterior salivary gland of the Dibranchiata. No ink-sac is present inNautilus.Fig.11.—Diagram to show the relations of the four nephridial sacs, the viscero-pericardial sac, and the heart and large vessels inNautilus(drawn by A.G. Bourne).neph,neph, on the right side point to the two nephridia of that side (the two of the opposite side are not lettered)—each is seen to have an independent aperture.xis the viscero-pericardial sac, the dotted line indicating its backward extension.visc. per. apert, marks an arrow introduced into the right aperture of the viscero-pericardial sac.r.e,r.e, point to the glandular enlarged walls of the afferent branchial vessels—two small glandular bodies of the kind are seen to project into each nephridial sac, whilst a larger body of the same kind depends from each of the four branchial afferent vessels into the viscero-pericardial sac.v.c, Vena-cava.vent, Ventricle of the heart.ao, Cephalic aorta (the small abdominal aorta not drawn).a.b.v, Branchial vessel.e.v.b, Efferent branchial vessel.Coelom, Blood-vascular System and Excretory Organs.—Nautilusand the other Cephalopoda conform to the general Molluscan characters in regard to these organs. Whilst the general visceral cavity forms a lacunar blood-system or series of narrow spaces, connected with the trunks of a well-developed vascular system, that part of the original coelom surrounding the heart and known as the Molluscan pericardium is shut off from this general blood-lymph system, and communicates, directly inNautilus, in the rest through the renal sacs, with the exterior. In the Cephalopoda this specialized pericardial cavity is particularly large, and has been recognized as distinct from the blood-carrying spaces, even by anatomists who have not considered the pericardial space of other Mollusca to be thus isolated. The enlarged pericardium, which may even take the form of a pair of sacs, has been variously named, but is best known as the viscero-pericardial sac or chamber. InNautilusthis sac occupies the whole of the postero-dorsal surface and a part of the antero-dorsal (see fig. 10,x), investing the genital and other viscera which lie below it, and having the ventricle of the heart suspended in it. Certain membranes forming incomplete septa, and a curious muscular band—the pallio-cardiac band—traverse the sac. The four branchial afferent veins, which in traversing the walls of the four renal sacs give off, as it were, glandular diverticula into those sacs, also give off at the same points four much larger glandular masses, which hang freely into the viscero-pericardial chamber (fig. 11,r.e). InNautilusthe viscero-pericardial sac opens to the exterior directly by a pair of apertures, one placed close to the right and one close to the left posterior renal aperture (fig. 5,visc. per). This direct opening of the pericardial sac to the exterior is an exception to what occurs in all other Mollusca. In all other Molluscs the pericardial sac opens into the renal organs, and through them or the one renal organ to the exterior. InNautilusthere is no opening from the viscero-pericardial sac into the renal sacs. Therefore the external pore of the viscero-pericardial sac may possibly be regarded as a shifting of the reno-pericardial orifice from the actual wall of the renal sac to a position alongside of its orifice. Parallel cases of such shifting are seen in the varying position of the orifice of the ink-bag in Dibranchiata, and in the orifice of the genital ducts of Mollusca, which in some few cases (e.g.Spondylus) open into the renal organs, whilst in other cases they open close by the side of the renal organs on the surface of the body. The viscero-pericardial sac of the Dibranchs is very large also, and extends into the dorsal region. It varies inshape—that is to say, in the extensions of its area right and left between the various viscera—in different genera, but in the Decapods is largest. In an extension of this chamber is placed the ovary ofSepia, whilst the ventricle of the heart and the branchial hearts and their appendages also lie in it. It is probable that water is drawn into this chamber through the renal sacs, since sand and other foreign matters are found in it. In all it opens into the pair of renal sacs by an orifice on the wall of each, not far from the external orifice (fig. 29,y, y′). There does not seem any room for doubting that each orifice corresponds to the reno-pericardial orifice which we have seen in the Gastropoda, and shall find again in the Lamellibranchia.Fig.12.—Diagram to show the relations of the heart in the Mollusca. (From Gegenbaur.)A, Part of the dorsal vascular trunk and transverse trunks of a worm.B, Ventricle and auricles ofNautilus.C, Of a Lamellibranch, ofChiton, or ofLoligo.D, OfOctopus.E, Of a Gastropod.a, Auricle.v, Ventricle.ac, Arteria = cephalica = (aorta).ai, Arteria abdominalis. The arrows show the direction of the blood-current.The circulatory organs, blood-vessels and blood ofNautilusdo not differ greatly from those of Gastropoda. The ventricle of the heart is a four-cornered body, receiving a dilated branchial efferent vessel (auricle) at each corner (fig. 11). It gives off a cephalic aorta anteriorly, and a smaller abdominal aorta posteriorly. The diagram, fig. 12, serves to show how this simple form of heart is related to the dorsal vessel of a worm or of an Arthropod, and how by a simple flexure of the ventricle (D) and a subsequent suppression of one auricle, following on the suppression of one branchia, one may obtain the form of heart characteristic of the anisopleurous Gastropoda (excepting the Aspidobranchia). The flexed condition of the heart is seen inOctopus, and is to some extent approached byNautilus, the median vessels not presenting that perfect parallelism which is shown in the figure (B). The most remarkable feature presented by the heart ofNautilusis the possession of four instead of two auricles, a feature which is simply related to the metamerism of the branchiae. By the left side of the heart ofNautilus, attached to it by a membrane, and hanging loosely in the viscero-pericardial chamber, is the pyriform sac of Owen. This has been shown to be the rudimentary left oviduct or sperm-duct, as the case may be (E.R. Lankester and A.G. Bourne), the functional right ovi-sac and its duct being attached by a membrane to the opposite side of the heart.The cephalic and abdominal aortae ofNautilusappear, after running to the anterior and posterior extremes of the animal respectively, to open into sinus-like spaces surrounding the viscera, muscular masses, &c. These spaces are not large, but confined and shallow. Capillaries are stated to occur in the integument. In the Dibranchs the arterial system is very much more complete; it appears in some cases to end in irregular lacunae or sinuses, in other cases in true capillaries which lead on into veins. An investigation of these capillaries in the light of modern histological knowledge is much needed. From the sinuses and capillaries the veins take origin, collecting into a large median trunk (the vena cava), which in the Dibranchs as well as inNautilushas a ventral (postero-ventral) position, and runs parallel to the long axis of the animal. InNautilusthis vena cava gives off at the level of the gills four branchial afferent veins (fig. 11,v.c.), which pass into the four gills without dilating. In the Dibranchs at a similar position the vena cava gives off a right and a left branchial afferent vein, each of which, traversing the wall of the corresponding renal sac and receiving additional factors, dilates at the base of the corresponding branchial plume, forming there a pulsating sac—the branchial heart. Attached to each branchial heart is a curious glandular body, which may possibly be related to the larger masses (fig. 11,r.e.) which depend into the viscero-pericardial cavity from the branchial afferent veins ofNautilus. From the dilated branchial heart the branchial afferent vessel proceeds, running up the adpallial face of the gill-plume. From each gill-plume the blood passes by the branchial efferent vessels to the heart, the two auricles being formed by the dilatation of these vessels.The blood contains the usual amoeboid corpuscles, and a diffused colouring matter—the haemocyanin of Fredericque—which has been found also in the blood ofHelix, and in that of the ArthropodsHomarusandLimulus. It is colourless in the oxidized, blue in the deoxidized state, and contains copper as a chemical constituent.The renal sacs and renal glandular tissue are closely connected with the branchial advehent vessels inNautilusand in the other Cephalopoda. The arrangement is such as to render the typical relations and form of a renal tube difficult to trace. In accordance with the metamerism ofNautilusalready noticed, there are two pairs of renal organs. Each assumes the form of a sac opening by a pore to the exterior. As is usual in renal tubes a glandular and a non-glandular portion are distinguished in each sac; these portions, however, are not successive parts of a tube, as happens in other cases, but they are localized areae of the wall of the sac. The glandular renal tissue is, in fact, confined to a tract extending along that part of the sac’s wall which immediately invests the great branchial afferent vein. The vein in this region gives off directly from its wall a complete herbage of little venules, which branch and anastomose with one another, and are clothed by the glandular epithelium of the renal sac. The secretion is accumulated in the sac and passed by its aperture to the exterior. Probably the nitrogenous excretory product is very rapidly discharged; inNautilusa pink-coloured powder is found accumulated in the renal sacs, consisting of calcium phosphate. The presence of this phosphatic calculus by no means proves that such was the sole excretion of the renal glandular tissue. InNautilusa glandular growth like that rising from the wall of the branchial vessel into its corresponding renal sac, but larger in size, depends from each branchial afferent vessel into the viscero-pericardial sac and forms the pericardial gland—probably identical with the “appendage” of the branchial hearts of Dibranchs.The chief difference, other than that of number, between the renal organs of the Dibranchs and those ofNautilus, is the absence of the accessory growths depending into the viscero-pericardial space just mentioned, and, of more importance, the presence in the former of a pore leading from the renal sac into the viscero-pericardial sac (y,y′in fig. 29). The external orifices of the renal organs are also more prominent in Dibranchs than inNautilus, being raised on papillae (npin fig. 29;rin fig. 25). InSepiathe two renal sacs give off each a diverticulum dorsalwards, which unites with its fellow and forms a great median renal chamber, lying between the ventral portions of the renal organs and the viscero-pericardial chamber. InLoligothe fusion of the two renal organs to form one sac is still more obvious, since the ventral portions are united. InOctopusthe renal sacs are quite separate.Gonads and Genital Ducts.—InNautilusit has been shown by E. Ray Lankester and A.G. Bourne that the genital ducts of both sexes are paired right and left, the left duct being rudimentary and forming the “pyriform appendage,” described by Sir R. Owen as adhering by membranous attachment to the ventricle of the heart, and shown by W. Keferstein to communicate by a pore with the exterior. The ovary (female gonad) or the testis (male gonad) lies inNautilus, as in the Dibranchs, in a distinct cavity walled off from the other viscera, near the centro-dorsal region. This chamber is formed by the coelomic or peritoneal wall; the space enclosed is originally part of the coelom, and inSepiaandLoligois, in the adult, part of the viscero-pericardial chamber. InOctopusit is this genital chamber which communicates by a right and a left canal with the renal sac, and is the only representative of pericardium. The ovary or testis is itself a growth from the inner wall of this chamber, which it only partly fills. InNautilusthe right genital duct, which is functional, is a simple continuation to the pore on the postero-dorsal surface of the membranous walls of the capsule in which lies the ovary or the testis, as the case may be. The gonad itself appears to represent a single median or bilateral organ.The ovary forms a large projection into the genital coelom, and the coelomic epithelium is deeply invaginated into the mass of the gonad, so as to constitute an ovarian cavity communicating with the coelom by a narrow aperture. The ova originate in the epithelium, migrate below it and then, as they enlarge, project into the ovarian cavity, pushing the epithelium before them. Each ovum is surrounded by a follicular epithelium which is nourished by numerous blood-vessels, and which penetrates into the surface of the ovum in numerous folds. When mature, the ovum is contained in a membrane or chorion with a micropyle, and escapes by dehiscence of the follicle into the genital coelom and duct. In its passage to the exterior the ovum passes a glandular structure on the wall of the genital capsule, which probably secretes the gelatinous substance enclosing the eggs. In addition to this internal gland there are other accessory glands, which are not related to the genital duct or sac but are differentiations of the wall of the pallial cavity, and occur on the inner wall of the pallium inNautilus, on the somatic wall in Dibranchiata. InNautilusthey form a continuous mass. These produce the external envelopes of the eggs.In the male the testis is a specialized portion of the wall of the genital coelom, and has a structure comparable to that of the ovary. The spermatozoa pass through an orifice from the cavity of the testis to the genital capsule, and thence to the spermiduct. The spermiduct is provided with a glandular pouch, and opens into a terminal reservoir known as Needham’s sac or the spermatophore sac. The function of this pouch is to form the spermatophore, which is an elastic tube formed of structureless secretion and invaginated into itself. The deeper part contains the spermatozoa, the external part is called the connective, and is usually much contracted and spirallycoiled. When the spermatophore is expelled into the water the connective is extended and evaginated, and the sac containing the sperms bursts. InNautilusthe spermatophore when uncoiled is a little over 30 mm. in length. These spermatophores are somewhat similar to those formed in certain pulmonate Gastropods.The eggs are laid shortly after copulation. InNautilusthey are laid separately, each being about 4 cm. long and contained in two thick shells, the outer of which is partly open.Fig.13.—Nervous system ofNautilus pompilius(from Genebaur, after Owen).t, t, Ganglion-like enlargements on nerves passing from the pedal ganglion to the inner series of tentacles.t′, Nerves to the tentacles of the outer or annular lobe.b, Pedal ganglion-paira, Cerebral ganglion-pair.c, Pleuro-visceral ganglionic band (fused pleural and visceral ganglion-pairs).d, Genital ganglion placed on the course of the large visceral nerve, just before it gives off its branchial and its osphradial branches.m, Nerves from the pleural ganglion to the mantle-skirt.Nervous System.—Nautilus, like the other Cephalopoda, exhibits a great concentration of the typical Molluscan ganglia, as shown in fig. 13. The ganglia take on a band-like form, and are but little differentiated from their commissures and connectives—an archaic condition reminding us ofChiton. The special optic outgrowth of the cerebral ganglion, the optical ganglion (fig. 13,o), is characteristic. The cerebral ganglion-pair (a) lying above the oesophagus is connected with two suboesophageal ganglion-pairs, of band-like form. The anterior of these is the pedalb, b, and supplies the circumoral lobes and tentacles, and the funnel, a fact which proves the pedal origin of these organs. The hinder band is the visceral and pleural pair fused; from its pleural portion nerves pass to the mantle, from its visceral portion nerves to the branchiae and genital ganglion (fig. 13,d), and in immediate connexion with the latter is a nerve to the osphradium or olfactory papilla. A labial commissure arises by a double root from the cerebral ganglia and gives off a stomatogastric commissure, which passes under the pharynx immediately behind the radula and bears a buccal ganglion on either side.Special Sense-Organs.—Nautiluspossesses a pair of osphradial papillae (fig. 4,olf) corresponding in position and innervation to Spengel’s organ placed at the base of the ctenidia (branchiae) in all classes of Mollusca. This organ has not been detected in other Cephalopoda.Nautiluspossesses other olfactory organs in the region of the head. Just below the eye is a small triangular process (not seen in our figures), having the structure of a shortened and highly-modified tentacle and sheath. By A. Valenciennes, who is followed by W. Keferstein, this is regarded as an olfactory organ. The large nerve which runs to this organ originates from the point of juncture of the pedal with the optic ganglion. The lamelliform organ upon the inner inferior tentacular lobe ofNautilusis possibly also olfactory in function. In Dibranchs behind the eye is a pit or open canal supplied by a nerve corresponding in origin to the olfactory nerve ofNautilusabove mentioned. Possibly the sense of taste resides in certain processes within the mouth ofNautilusand other Cephalopoda.The otocysts ofNautiluswere discovered by J.D. Macdonald. Each lies at the side of the head, ventral to the eye, resting on the capito-pedal cartilage, and supported by the large auditory nerve which apparently arises from the pedal ganglion but originates in the cerebral. It has the form of a small sac, 1 to 2 mm. in diameter, and contains whetstone-shaped crystals, such as are known to form the otoliths of other Mollusca.The eye ofNautilusis among the most interesting structures of that remarkable animal. No other animal which has the same bulk and general elaboration of organization has so simple an eye as that ofNautilus. When looked at from the surface no metallic lustre, no transparent coverings, are presented by it. It is simply a slightly projecting hemispherical box like a kettle-drum, half an inch in diameter, its surface looking like that of the surrounding integument, whilst in the middle of the drum-membrane is a minute hole (fig. 3,u). Sir R. Owen very naturally thought that some membrane had covered this hole in life, and had been ruptured in the specimen studied by him. It, however, appears from the researches of V. Hensen that the hole is a normal aperture leading into the globe of the eye, which is accordingly filled by sea-water during life. There is no dioptric apparatus inNautilus, and in place of refracting lens and cornea we have actually here an arrangement for forming an image on the principle of “the pin-hole camera.” There is no other eye known in the whole animal kingdom which is so constructed. The wall of the eye-globe is tough, and the cavity is lined solely by the naked retina, which is bathed by sea-water on one surface and receives the fibres of the optic nerve on the other (see fig. 14, A). As in other Cephalopods (e.g.fig. 33,Ri, Re, p), the retina consists of two layers of cells, separated by a layer of dark pigment. The most interesting consideration connected with this eye ofNautilusis found when the further facts are noted—(1) that the elaborate lens-bearing eyes of Dibranchiata pass through a stage of development in which they have the same structure as the eye ofNautilus—namely, are open sacs (fig. 34); and (2) that amongst other Mollusca examples of cephalic eyes can be found which in the adult condition are, like the eye ofNautilusand the developing eye of Dibranchs, simple pits of the integument, the cells of which are surrounded by pigment and connected with the filaments of an optic nerve. Such is the structure of the eye of the limpet (Patella), and in such a simple eye we obtain the clearest demonstration of the fact that the retina of the Molluscan cephalic eye, like that of the Arthropod cephalic eye and unlike that of the vertebrate myelonic eye, is essentially a modified area of the general epiderm, and that the sensitiveness of its cells to the action of light and their relation to nerve-filaments is only a specialization and intensifying of a property common to the whole epiderm of the surface of the body. What, however, strikes us as especially remarkable is that the simple form of a pit, which inPatellaserves to accumulate a secretion which acts as a refractive body, should inNautilusbe glorified and raised to the dignity of an efficient optical apparatus. In all other Mollusca, starting as we may suppose from the follicular or pit-like condition, the eye has proceeded to acquire the form of aclosedsac, the cavity of the closed vesicle being then filled partially or completely by a refractive body (lens) secreted by its walls (fig. 14, B). This is the condition attained in most Gastropoda. It presents a striking contrast to the simple Arthropod eye, where, in consequence of the existence of a dense exterior cuticle, the eye does not form a vesicle, and the lens is always part of that cuticle.Fig.14.—Diagrams of Sections of the Eyes of Mollusca.A,Nautilus(andPatella).B, Gastropod (LimaxorHelix).C, Dibranchiate Cephalopod (Oigopsid).Pal, Eyelid (outermost fold).Co, Cornea (second fold).Ir, Iris (third fold).Int1,2,3,4, Different parts of the integument.l, Deep portion of the lens.l1, Outer portion of the lensCo.ep, Ciliary body.R, Retina.N.op, Optic nerve.G.op, Optic ganglion.x, Inner layer of the retina.N.S., Nervous stratum of the retina. (From Balfour, after Grenacher.)The development ofNautilusis still entirely unknown. Dr Arthur Willey, during his sojourn in the East Indies, made special efforts to obtain fertilized eggs, both by offering rewards to the native fishermen and collectors and by keeping the living adults in captivity, but without success.Phylogeny and Classification.—AsNautilusis the only living genus of the Tetrabranchiata, our knowledge of all the rest is based upon the study of their fossil shells. A vast number of species of shell similar in structure to that ofNautilusare known, chiefly from Primary and Secondary formations. These are divided into two sub-orders by differences in the form and structure of the initial chamber. In the Nautiloidea this chamber has the form of an obtuse cone, on the apex of which is a slit-like mark or cicatrix, elongated dorso-ventrally and placed opposite to the blind end of the siphuncle, which indents the front wall of the initial chamber but does not enter its cavity. In the Ammonoidea, on the other hand, the initial chamber is inflated, and is spheroidal, oval or pyriform in shape, with no cicatrix, and separated from the first air-chamber by a constriction. The siphuncle also commences with a dilatation which deeply indents the front wall of the initial chamber, called the protoconch, but does not penetrate into its cavity. Munier-Chalmas has shown that the cavity of the protoconch is traversed by a tubular organ, the “prosiphon,” which does not communicate with the true siphuncle, the place of which it is supposed to take in the early life of the animal. It is generally held, as suggested byAlpheus Hyatt, that the initial chamber of the Nautiloidea corresponds not to the protoconch of the Ammonoids, but to the second chamber of the latter, and that there existed in the young Nautiloids a true initial chamber, a protoconch which was either uncalcified or deciduous. The shell of the living nautilus does not decide this question, as its early stages are unknown, and there is a little vacuity in the centre of the spirally coiled shell which may have been originally occupied by the true protoconch.The septa in the Nautiloidea are generally concave towards the aperture of the shell, their curvature therefore directed backwards (fig. I); in the Ammonoidea, on the other hand, the convexity is usually towards the aperture, the curvature therefore directed forwards. The lines along which the edges of the septa are united to the shell are known as “sutures,” and these in the Nautiloidea are simply curved or slightly lobed, whereas in the Ammonoidea they are folded in various degrees of complexity; the projections of the suture towards the mouth of the shell are called saddles, those in the opposite direction lobes. The siphuncle in theNautiluspierces the centres of the septa, and in fossil Nautiloids it is usually central or sub-central. In a few cases it is marginal, and in that case may be external,i.e.ventral, or internal,i.e.dorsal. In Ammonoids the siphuncle is always marginal, and usually external. Its walls in the livingNautilusare strengthened by the deposit of calcareous granules, and in some fossil forms the wall is completely calcified. But this proper calcified wall is quite distinct from calcareous tubes surrounding the siphuncle, which are developed from the septa. In the pearly nautilus each septum is prolonged backwards at the point where it is pierced by the siphuncle, forming a shelly tube somewhat like the neck of a bottle. In many fossil forms these septal necks are continued from the septum from which they arise to the next, so that the siphuncle is enclosed in a complete secondary calcareous tube. In the majority of Nautiloids the septal necks are directed backwards, and they are said to be retrosiphonate. In the majority of the Ammonoids the septal necks are continued forwards from the septa to which they belong, and such forms are termed prosiphonate.The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic periods. The Nautiloidea are the most ancient, appearing first in the Upper Cambrian, the genera being most numerous in the Palaeozoic period, and comparatively few surviving into the Secondary. On the other hand, the Ammonoidea are scarce in Palaeozoic formations, being represented in deposits earlier than the Carboniferous only by comparatively simple types, such asClymeniaandGoniatites. In the Secondary period Ammonoids were very abundant, both in genera and species and in individuals, and with few local exceptions none are known to have survived even to the commencement of the Tertiary. In the widest sense the genusNautilushas existed since the Ordovician (Silurian) period, but the oldest types are not properly to be placed in the same genus as the existing form. Even with this qualification the genus is very ancient, shells very similar to those of the livingNautilusbeing found in the Upper Cretaceous.It has been maintained by some zoologists that the Ammonoidea were Dibranchiate, though it would not follow from this that the shell was, therefore, internal. They are, however, generally classed with the Tetrabranchiata, and the absence of all evidence of the possession of an ink-sac is in favour of this view. There can be little doubt that they gave rise to the Dibranchiata.About 2500 fossil species are included in the Nautiloidea, but only a few species of the genusNautilussurvive. Some of the fossil forms are very large, the shell reaching a length of 2 metres, or 6 ft. 6 in. Of the Ammonoidea more than 5000 species have been described, and some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in diameter.Associated with various forms of Ammonoids there have been found peculiar horny or calcified plates, sometimes contained within the body-chamber of the shell, sometimes wholly detached. The most typical form of these structures has been namedaptychus.It consists of two bilaterally symmetrical halves, of somewhat semicircular shape, and attached to one another by their straight inner margins, like a pair of doors. In some cases the aptychus is thin and horny, but more often it is thick and calcified, in which case the principal layer has a peculiar cellular structure. The surface may be smooth or sculptured, and one side is usually marked by concentric lines of growth. Another type is similar, except that the two halves are united in the middle line; bodies of this character are calledsynaptychus; they occur in the body-chamber of species ofScaphites. Another form calledanaptychusconsists of a thin horny undivided plate which is concentrically striated. This is associated with species ofAmmonitesandGoniatites.Many theories have been proposed in explanation of these structures. According to Sir Richard Owen, the aptychus is an operculum developed in a part of the body corresponding to the hood ofNautilus. E. Ray Lankester suggested that the double plate was borne on the surface of the nidamental gland, with the form and sculpturing of which inNautilusit closely agrees. On this view the aptychus would occur only in females. The most recent view is that these structures could not have been opercula because of their constant position inside the body-chamber, and that they were not external secretions at all, but a calcified internal cartilage situated at the base of the funnel.Classification of Tetrabranchiata.—Cephalopoda in which the mantle is entirely enclosed by a multilocular siphunculated shell, which may or may not be coiled. Only the last compartment of the shell occupied by the body of the animal. Numerous pedal tentacles around the mouth, which are retractile within sheaths. Halves of the funnel not united. Two pairs of ctenidia, and two pairs of renal tubes without reno-pericardial apertures. Pericardium opens directly to exterior. Cephalic cartilage wholly ventral. Optic vesicles with apertures, without crystalline lens.Sub-order1.Nautiloidea.—Initial chamber not inflated, with dorso-ventral cicatrix at extremity.Fam. 1.Orthoceratidae. Shell straight or slightly curved, with a simple aperture, large terminal chamber and cylindrical siphuncle.Orthoceras, Silurian to Trias.Baltoceras, Silurian.Fam. 2.Actinoceratidae. Shell straight or slightly curved, with wide siphuncle contracted at level of septa.Actinoceras, Silurian to Carboniferous.Discosorus, Silurian.Huronia, Silurian.Loxoceras, Silurian to Carboniferous.Fam. 3.Endoceratidae. Shell straight, with wide margina siphuncle, necks produced into tubes fitting into one another.Endoceras, Silurian.Fam. 4.Gomphoceratidae. Shell globular, straight or arcuate, aperture contracted.Gomphoceras, Silurian.Phragmoceras, Silurian.Fam. 5.Ascoceratidae. Shell straight, ampulliform, summit truncate, terminal chamber extending nearly whole length of shell ventrally.Ascoceras, Silurian.Glossoceras, Silurian.Fam. 6.Poterioceratidae. Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa.Poterioceras, Silurian to Carboniferous.Streptoceras, Silurian.Fam. 7.Cyrtoceratidae. Shell slightly curved, aperture simple, siphuncle wide, septa approximated.Cyrtoceras, Devonian.Fam. 8.Lituitidae. Shell coiled in one plane with the terminal part uncoiled, aperture contracted.Lituites, Silurian.Ophidioceras, Silurian.Fam. 9.Trochoceratidae. Shell helicoidally coiled, dextral or sinistral, the last whorl generally uncoiled.Trochoceras, Devonian.Adelphoceras, Devonian.Fam. 10.Nautilidae. Shell coiled in one plane, aperture wide and simple, siphuncle central.Nautilus, recent.Trocholites, Silurian.Gyroceras, Silurian to Carboniferous.Hercoceras, Silurian.Ptenoceras, Devonian.Discites, Carboniferous.Fam. 11.Bactritidae. Shell straight, conical, siphuncle narrow and marginal, necks long, infundibuliform, sutures undulating.Bactrites, Silurian and Devonian.Sub-order2.Ammonitoidea,—Initial chamber spheroidal; siphuncle narrow and simple; septa convex towards aperture; sutures complex.Tribe1.Retrosiphonata.—Siphuncular necks projecting behind the septa as in Nautiloidea. Sutures form simple undulations. Occur exclusively in Palaeozoic strata from Devonian upwards.Fam. 1.Goniatitidae. Shell nautiloid, with simple sutures and ventral siphuncle.Goniatites, Devonian and Carboniferous.Anarcestes, Devonian.Fam. 2.Clymeniidae. Shell nautiloid, with simple sutures, siphuncle dorsal, that is, internal.Clymenia, Upper Devonian.Tribe2.Prosiphonata.—Siphuncular necks projecting in front of the septa. Sutures form deeply indented lobes and saddles.Fam. 1.Arcestidae. Globular and smooth or nearly smooth, with reduced umbilicus, terminal chamber very deep, an aptychus present.Popanoceras, Permian.Cyclolobus, Permian,Arcestes, Trias.Lobites, Trias.Fam. 2.Tropitidae. Shells globular, but having radiating and tuberculated costae.Thalassoceras, Permian.Tropites, Trias.Sibirites, Trias.Fam. 3.Ceratitidae. Shells coiled, with a large umbilicus, terminal chamber short, sutures with simple saddles.Trachyceras, Upper Trias.Ceratites, Trias.Dinarites, Trias.Some genera with helicoidal shells are related to these coiled forms, viz.Cochloceras, Trias; also some straight forms,e.g. Rhab-doccras, Trias.Fam. 4.Pinacoceratidae. Shell compressed, smooth, terminal chamber short, sutures very complicated, convex.Pinacoceras, Trias.Fam. 5.Phylloceratidae. Shell coiled, the whorls overlapping each other, sutures formed of numerous lobes and saddles.Phytloceras, Jurassic.Fam. 6.Lytoceratidae. Shell discoid, whorls loosely united or uncoiled, sutures deeply indented, but with only three saddles and lobes.Lytoceras, Jurassic and Cretaceous.Macroscaphites, Cretaceous.Reunites, Cretaceous.Ptychoeeras, Cretaceous.Turrilites, Cretaceous.Baculites, Cretaceous.Fam. 7.Ammonitidae. Shell coiled, with narrow whorls which do not embrace one another, aperture simple, a horny anaptychus present.Ammonites, Jurassic.Arietites, Jurassic.Aegoceras, Lias.Fam. 8.Harpoceratidae. Shell discord and flattened, with a carinated border, aperture provided with lateral projections,a calcareous aptychus, formed of two pieces.Harpoceras, Jurassic.Oppelia, Jurassic.Lissoceras, Jurassic and Cretaceous.Fam. 9.Amaltheidae. Shell flattened, with a prominent carina continued anteriorly into a rostrum.Amaltheus, Lias.Cardioceras, Jurassic.Schloenbachia, Cretaceous.Fam. 10.Stephanoceratidae. Shell not carinated, but with radiating costae, which are often bifurcated, aperture often with lateral projections which contract it, aptychus formed of two pieces.Stephanoceras, Morphoceras, Pensphinctes, Peltoceras, Jurassic.Hoplites, Cretaceous.Acanthoceras, Cretaceous.Cosmoceras, Jurassic. Various more or less uncoiled forms are related to this family, viz.Scaphites, Crioceras, Cretaceous.
Head, Foot, Mantle-skirt and Sub-pallial Chamber.—In the pearly nautilus the ovoid visceral hump is completely encircled by the free flap of integument known as mantle-skirt (figs. 2, 3,d, e). In the antero-dorsal region this flap is enlarged so as to be reflected a little over the coil of the shell which rests on it. In the postero-ventral region the flap is deepest, forming an extensive sub-pallial chamber, at the entrance of which e is placed in fig. 3. A view of the interiorof the sub-pallial chamber, as seen when the mantle-skirt is retroverted and the observer faces in the direction indicated by the reference line passing from e in fig. 3, is given in fig. 4. With this should be compared the similar view of the sub-pallial chamber of the DibranchiateSepia. It should be noted as a difference betweenNautilusand the Dibranchiates that in the former the nidamental gland (in the female) lies on that surface of the pallial chamber formed by the dependent mantle-flap (fig. 4,g.n.; fig. 1, V), whilst in the latter it lies on the surface formed by the body-wall; in fact in the former the base of the fold forming the mantle-skirt comprises in its area a part of what is unreflected visceral hump in the latter.
m, The dorsal “hood” formed by the enlargement of the outer or annular lobe of the fore-foot, and corresponding to the sheaths of two tentacles (g, gin fig. 6).
n, Tentacular sheaths of lateral portion of the annular lobe.
u, The left eye.
b, The nuchal plate, continuous at its right and left posterior angles with the root of the mid-foot, and corresponding to the nuchal cartilage of Sepia.
c, Visceral hump.
d, The free margin of the mantle-skirt, the middle letterdpoints to that portion of the mantle-skirt which is reflected over a part of the shell as seen in fig. 1,b; the cup-like fossa to which b and d point in the present figure is occupied by the coil of the shell.
g.a.points to the lateral continuation of the nuchal plate b to join the root of the mid-foot or siphon.
e, points to the concave margin of the mantle-skirt leading into the sub-pallial chamber.
g, The mid-foot or siphon.
k, The superficial origin of its retractor muscles closely applied to the shell and serving to hold the animal in its place.
l, The siphuncular pedicle of the visceral hump broken off short.
v, v, The superior and inferior ophthalmic tentacles.
The apertures of the two pairs of renal sacs, of the viscero-pericardial sac, of the genital ducts, and of the anus, are shown in position on the body-wall of the pallial chamber ofNautilusin figs. 4, 5. There are nine apertures in all, one median (the anus) and four paired. Besides these apertures we noticetwopairs of gill-plumes which are undoubtedly typical ctenidia, and a short papilla (the osphradium) between each anterior and posterior gill-plume (see figs. 4, 5, and explanation). As compared with this in a Dibranchiate, we find (fig. 25) only four apertures, viz. the median anus with adjacent orifice of the ink-sac, the single pair of renal apertures, and one asymmetrical genital aperture (on the left side) except in female Octopoda and a few others, where the genital ducts and their apertures are paired. No viscero-pericardial pores are present on the surface of the pallial chamber, since in the Dibranchiata the viscero-pericardial sac opens by a pore into each nephridium instead of directly to the surface. A single pair of ctenidia (gill-plumes) is present instead of the two pairs inNautilus. The existence of two pairs of ctenidia and of two pairs of renal sacs inNautilus, placed one behind the other, is highly remarkable. The interest of this arrangement is in relation to the general morphology of the Mollusca, for it is impossible to view this repetition of organs in a linear series as anything else than an instance of metameric segmentation, comparable to the segmentation of the ringed worms and Arthropods. The only other example which we have of this metamerism in the Mollusca is presented by the Chitons. There we find not two pairs of ctenidia merely, but sixteen pairs (in some species more) accompanied by a similar metamerism of the dorsal integument, which carries eight shells. InChitonthe renal organs are not affected by the metamerism as they are inNautilus. It is impossible on the present occasion to discuss in the way which their importance demands the significance of these two instances among Mollusca of incomplete or partial metamerism; but it would be wrong to pass them by without insisting upon the great importance which the occurrence of these isolated instances of metameric segmentation in a group of otherwise unsegmented organisms possesses, and the light which they may be made to throw upon the nature of metameric segmentation in general.
a, Muscular band passing from the mid-foot to the integument.
b, The valve on the surface of the funnel, partially concealed by the inrolled lateral margin of the latter.
c, The mantle-skirt retroverted.
an, The median anus.
x, Post-anal papilla of unknown significance.
g.n., Nidamental gland.
r.ov, Aperture of the right oviduct.
l.ov, Aperture of the rudimentary left oviduct (pyriform sac of Owen).
neph.a, Aperture of the left anterior renal sac.
neph.p, Aperture of the left posterior renal sac.
viscper, Left aperture of the viscero-pericardial sac.
olf, The left osphradium placed near the base of the anterior gill-plume.
The foot and head ofNautilusare in the adult inextricably grown together, the eye being the only part belonging primarily to the head which projects from the all-embracing foot. The fore-foot or front portion of the foot has the form of a number of lobes carrying tentacles and completely surrounding the mouth (figs. 2, 3). The epipodia incline towards each other posteriorly so as to form an incomplete siphon (fig. 4), a condition which is completed and rendered permanent in the tubular funnel of Dibranchiata. The epipodial nature of the funnel is well seen in young embryos, in which this organ is situated laterally and posteriorly between the mantle and the foot.
The lobes of the fore-foot ofNautilusand of the other Cephalopoda require further description. It has been doubted whether these lobes were rightly referred (by T.H. Huxley) to the fore-foot, and it has been maintained by some zoologists (H. Grenadier, H. vonJhering) that they are truly processes of the head. It appears to be impossible to doubt that the lobes in question are the fore-portion of the foot, when their development is examined (see fig. 35), further, when the fact is considered that they are innervated by the pedal ganglion. The fore-foot ofNautiluscompletely surrounds the buccal cone (fig. 6, e), so as to present an appearance with its expanded tentacles similar to that of the disk of a sea-anemone (Actinia). A.G. Bourne, of University College, prepared from actual specimens the drawings of this part in the male and femaleNautilusreproduced in fig. 6, and restored the parts to their natural form when expanded. The drawings show very strikingly the difference between male and female. In the females (lower figure), we observe in the centre of the disk the buccal cone e carrying the beak-like pair of jaws which project from the finely papillate buccal membrane. Three tentaculiferous lobes of the fore-foot are in immediate contact with this buccal cone; they are the right and left (c, c) inner lobes, and the inferior inner lobe (d)—called inferior because it really lies ventralwards of the mouth. This inner inferior lobe is clearly a double one, representing a right and left inner inferior lobe fused into one. A lamellated organ on its surface, known as Owen’s organ, probably olfactory in function (n), marks the separation of the constituent halves of this double lobe. Each half carries a group of fourteen tentacles. The right and the left inner lobes (c, c) each carry twelve tentacles. External to these three lobes the muscular substance of the mouth-embracing foot is raised into a wide ring, which becomes especially thick and large in the dorsal region where it is notably modified in form, offering a concavity into which the coil of the shell is received, and furnishing a protective roof to the retracted mass of tentacles. This part of the external annular lobe of the fore-foot is called the “hood” (figs. 2, 3,m). The median antero-posterior line traversing this hood exactly corresponds to the line of concrescence of the two halves of the fore-foot, which primitively grew forward one on each side of the head, and finally fused together along this line in front of the mouth. The tentacles carried by the great annular lobe are nineteen on each side, thirty-eight in all. They are called “digital,” and are somewhat larger than the “labial” tentacles carried on the three inner lobes. The dorsalmost pair of tentacles (markedgin fig. 6) are the only ones which actually belong to that part of the disk which forms the great dorsal hood m. The hood is, in fact, to a large extent formed by the enlarged sheaths of these two tentacles. All the tentacles of the circumoral disk are set in remarkable tubular sheaths, into which they can be drawn. The sheaths of some of those belonging to the external or annular lobe are seen in fig. 3, marked n. The sheaths are muscular as well as the tentacles, and are simply tubes from the base of which the solid tentacle grows. The functional significance of this sheathing arrangement is as obscure as its morphological origin. With reference to the latter, it appears highly probable that the tubular sheath represents the cup of a sucker such as is found on the fore-foot of the Dibranchiata. In any case, it seems to the writer impossible to doubt that each tentacle, and its sheath on a lobe of the circumoral disk of Nautilus, corresponds to a sucker on such a lobe of a Dibranchiate. W. Keferstein follows Sir R. Owen in strongly opposing this identification, and in regarding such tentacle as the equivalent of a whole lobe or arm of a Decapod or Octopod Dibranch. The details of these structures, especially in the facts concerning the hectocotylus and spadix, afford the most conclusive reasons for dissenting from Owen’s view. On the ventral side an extensive part of the internal surface of the muscular ring is laminated, forming the so-called “organ of Valenciennes,” peculiar to the female and serving for the attachment of the spermatophores. We have so far enumerated in the female nautilus ninety tentacles. Four more remain which have a very peculiar position, and almost lead to the suggestion that the eye itself is a modified tentacle. These remaining tentacles are placed one above (before) and one below (behind) each eye, and bring up the total to ninety-four (fig. 3v, v).
a, The shell.
b, Theouterring-like expansion (annular lobe) of the circumoral muscular mass of the fore-foot, carrying nineteen tentacles on each side—posteriorly this is enlarged to form the “hood” (markedvin fig. 1 andmin figs. 2 and 3). giving off the pair of tentacles markedgin the present figure.
c, The right and left inner lobes of the fore-foot, each carrying twelve tentacles in the female, in the male subdivided intop, the “spadix” or hectocotylus on the left side, andq, the “anti-spadix,” a group of four tentacles on the right side—it is thus seen that the subdivided right and left inner lobes of the male correspond to the undivided right and left inner lobes of the female.
d, The inner inferior lobe of the fore-foot, a bilateral structure in the female carrying two groups, each of fourteen tentacles, separated from one another by a lamellated organn, supposed to be olfactory in function—in the male the inner inferior lobe of the fore-foot is very much reduced, and has the form of a paired group of lamellae (din the upper figure).
e, The buccal cone, rising from the centre of the three inner lobes, and fringing the protruded calcareous beaks or jaws with a series of minute papillae.
f, The tentacles of the outer circumoral lobe or annular lobe of the fore-foot projecting from their sheaths.
g, The two most posterior tentacles of this series belonging to that part of the annular lobe which forms the hood (min figs. 2 and 3).
i, Superior ophthalmic tentacle.
k, Inferior ophthalmic tentacle.
l, Eye.
m, Paired laminated organ on each side of the base of the inner inferior lobe (d) of the female.
n, Olfactory lamellae upon the inner inferior lobe (in the female).
o, The siphon (mid-foot).
p, The spadix (in the male), the hectocotylized portion of the left inner lobe of the fore-foot representing four modified tentacles, eight being left unmodified.
q, The anti-spadix (in the male), being four of the twelve tentacles of the right inner lobe of the fore-foot isolated from the remaining eight, and representing on the right side the differentiated spadix of the left side. The four tentacles of the anti-spadix are set, three on one base and one on a separate base.
In the adult male nautilus we find the following important differences in the tentaculiferous disk as compared with the female (see upper drawing in fig. 6). The inner inferior lobe is rudimentary, and carries no tentacles. It is represented by three groups of lamellae (d), which are not fully exposed in the drawing. The right and left inner lobes are subdivided each into two portions. The right shows a larger portion carrying eight tentacles, and smaller detached groups (q) of four tentacles, of which three have their sheaths united whilst one stands alone. These four tentacles may be called the “anti-spadix.” The left inner lobe shows a similar larger portion carrying eight tentacles, and a curious conical body behind it corresponding to the anti-spadix. This is the “spadix.” It carries no tentacles, but is terminated by imbricated lamellae. These lamellae appear to represent the four tentacles of the anti-spadix of the right internal lobe, and are generally regarded as corresponding to that modification of the sucker-bearing arms of male Dibranchiate Siphonopods to which the name “hectocotylus” is applied. The spadix is in fact the hectocotylized portion of the fore-foot of the male nautilus. The hectocotylized arm or lobe of male Dibranchiata is connected with the process of copulation, and in the male nautilus the spadix has probably a similar significance, though it is not possible to suggest how it acts in this relation. It is important to observe that the modification of the fore-foot in the male as compared with the female nautilus is not confined to the existence of the spadix. The anti-spadix and the reduction of the inner inferior lobe are also male peculiarities. The external annular lobe in the male does not differ from that of the female; it carries nineteen tentacles on each side. The four ophthalmic tentacles are also present. Thus in the male nautilus we find altogether sixty-two tentacles, the thirty-two additional tentacles of the female being represented by lamelliform structures.
Musculature, Fins and, Cartilaginous Skeleton.—Without entering into a detailed account of the musculature ofNautilus, we may point out that the great muscular masses of the fore-foot and of the mid-foot (siphon) are ultimately traceable to a large transverse mass of muscular tissue, the ends of which are visible through the integument on the right and left surfaces of the body dorsal of the free flap of the mantle-skirt (fig. 1,l,l, and fig. 3,k). These muscular areae have a certain adhesion to the shell, and serve both to hold the animal in its shell and as the fixed supports for the various movements of the tentaculiferous lobes and the siphon. They are to be identified with the ring-like area of adhesion by which the foot-muscle of the limpet is attached to the shell of that animal. In the Dibranchs a similar origin of the muscular masses of the fore-foot and mid-foot from the sides of the shell—modified, as this is, in position and relations—can be traced.
Fig.7.—Minute structure of the cartilage ofLoligo(from Gegenbaur, after Furbringer)
a, Simple cells.
b, Dividing cells.
c, Canaliculi.
d, An empty cartilage capsule with its pores.
e, Canaliculi in section.
InNautilusthere are no fin-like expansions of the integument, whereas such occur in the Decapod Dibranchs along the sides of the visceral hump (figs. 15, 16). As an exception among Octopoda lateral fins occur inPinnoctopus(fig. 38, A), and inCirrhoteuthis(fig. 38, D).
InNautilusthere is a curious plate-like expansion of integument in the mid-dorsal region just behind the hood, lying between that structure and the portion of mantle-skirt which is reflected over the shell. This is shown in fig. 2,b. If we trace out the margin of this plate we find that it becomes continuous on each side with the sides of the funnel. InSepiaand other Decapods (not in Octopods) a closely similar plate exists in an exactly corresponding position (seebin figs. 10, 26). InSepiaa cartilaginous development occurs here immediately below the integument forming the so-called “nuchal plate,” drawn in fig. 8, D. The morphological significance of this nuchal lamella, as seen both inNautilusand inSepia, is not obvious. Cartilage having the structure shown in fig. 7 occurs in various regions of the body of Cephalopoda. In all Glossophorous Mollusca the lingual apparatus is supported by internal skeletal pieces, having the character of cartilage; but in the Cephalopoda such cartilage has a wider range.
InNautilusa large H-shaped piece of cartilage is found, forming the axis of the funnel (fig. 8, A, B). Its hinder part extends up into the head and supports the peri-oesophageal nerve-mass (a), whilst its two anterior rami extend into the tongue-like siphon. InSepia, and Dibranchs generally, the cartilage takes a different form, as shown in fig. 8, C. The processes of this cartilage cannot be identified in any way with those of the capito-pedal cartilage ofNautilus. The lower larger portion of this cartilage inSepiais called the cephalic cartilage, and forms a complete ring round the oesophagus; it completely invests also the ganglionic nerve-collar, so that all the nerves from the latter have to pass through foramina in the cartilage. The outer angles of this cartilage spread out on each side so as to form a cup-like receptacle for the eyes. The two processes springing right and left from this large cartilage in the median line (fig. 8, C) are the “pre-orbital cartilages”; in front of these, again, there is seen a piece like an inverted T, which forms a support to the base of the “arms” of the fore-foot, and is the “basi-brachial” cartilage. The Decapod Dibranchs have, further, the “nuchal cartilage” already mentioned, and inSepia, a thin plate-like “sub-ostracal” or (so-called) dorsal cartilage, the anterior end of which rests on and fits into the concave nuchal cartilage. In Octopoda there is no nuchal cartilage, but two band-like “dorsal cartilages.” In Decapods there are also two cartilaginous sockets on the sides of the funnel—“siphon-hinge cartilages”—into which fleshy knobs of the mantle-skirt are loosely fitted. InSepia, along the whole base-line of each lateral fin of the mantle (fig. 15), is a “basi-pterygial cartilage.” It is worthy of remark that we have, thus developed, in Dibranch Cephalopods a more complete internal cartilaginous skeleton than is to be found in some of the lower vertebrates. There are other instances of cartilaginous endo-skeleton in groups other than the Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms a skeletal support for the gill-plumes, whilst in the Arachnids (Mygale,Scorpio) and inLimulusa large internal cartilaginous plate—the ento-sternite—is developed as a support for a large series of muscles.
A, Capito-pedal cartilage ofNautilus pompilius.
apoints to the ridge which supports the pedal portion of the nerve-centre.
B, Lateral view of the same—the large anterior processes are sunk in the muscular substance of the siphon.
C, Cephalic cartilages ofSepia officinalis.
D, Nuchal cartilage ofSepia officinalis.
Alimentary Tract.—The buccal cone ofNautilusis terminated by a villous margin (buccal membrane), surrounding the pair of beak-like jaws, of which the ventral projects over the dorsal. These are very strong and dense inNautilus, being calcified. Fossilized beaks of Tetrabranchiata are known under the name of rhyncholites. In Dibranchs the beaks are horny, but similar in shape to those ofNautilus. They resemble in general those of a parrot, the lower beak being the larger and overlapping the upper or dorsal beak. The lingual ribbon and odontophoral apparatus have the structure which is typical for Glossophorous Mollusca. In fig. 9, A is represented a single row of teeth from the lingual ribbon ofNautilus, and in fig. 9, B, C, of other Cephalopoda.
InNautilusa long and wide crop or dilated oesophagus (fig. 10,cr) passes from the muscular buccal mass, and at the apex of the visceral hump passes into a highly muscular stomach, resembling the gizzard of a bird (fig 10,gizz). A nearly straight intestine passes from the muscular stomach to the anus, near which it develops a small caecum. In other Cephalopods the oesophagus is usually narrower and the muscular stomach more capacious, whilst a very important feature in the alimentary tract is formed by the caecum. In all butNautilusthe caecum lies near the stomach, and may be very capacious—much larger than the stomach inLoligo vulgaris—or elongated into a spiral coil. The simple U-shaped flexure of the alimentary tract, as seen in fig. 10, is the only important one which it exhibits in the Cephalopoda. The acini of the large liver ofNautilusare compacted into a solid reddish-brown mass by a firmmembrane, as also is the case in the Dibranchiata. The liver has four paired lobes inNautilus, which open by two bile-ducts into the alimentary canal at the commencement of the intestine. The bile-ducts unite before entering the intestine. In Dibranchiata the two large lobes of the liver are placed antero-dorsally (beneath the shell in Decapoda), and the bile-ducts open into the caecum. Upon the bile-ducts in Dibranchiata are developed yellowish glandular diverticula, which are known as “pancreas,” though neither physiologically nor morphologically is there any ground for considering either the so-called liver or the so-called pancreas as strictly equivalent to the glands so denominated in the Vertebrata. InNautilusthe equivalents of the pancreatic diverticula of the Dibranchs can be traced upon the relatively shorter bile-ducts.
a, The shell.
b, The nuchal plate, identical with the nuchal cartilage ofSepia(see fig. 2,b).
c, The integument covering the visceral hump.
d, The mantle flap or skirt in the dorsal region where it rests against the coil of the shell.
e, The inferior margin of the mantle-skirt resting on the lip of the shell represented by the dotted line.
f, The pallial chamber with two of the four gills.
g, The vertically cut median portion of the mid-foot (siphon).
h, The capito-pedal cartilage (see fig. 8).
i, The valve of the siphon.
l, The siphuncular pedicle (cut short).
m, The hood or dorsal enlargement of the annular lobe of the fore-foot.
n, Tentacles of the annular lobe.
p, Tentacles of inner inferior lobe.
q, Buccal membrane.
r, Upper jaw or beak.
s, Lower jaw or beak.
t, Lingual ribbon.
x, The viscero-pericardial sac.
n.c, Nerve-collar.
oe, Oesophagus.
cr, Crop.
gizz, Gizzard.
int, Intestine.
an, Anus.
nept, Aperture of a nephridial sac.
r.e, Renal glandular masses on the walls of the afferent branchial veins (see fig. 11).
a.b.v, Afferent branchial vessel.
e.b.v, Efferent branchial vessel.
vt, Ventricle of the heart.
Posterior salivary glands are not developed inNautilus, but on each side in the wall of the buccal mass is a gland corresponding to the anterior salivary gland of the Dibranchiata. No ink-sac is present inNautilus.
neph,neph, on the right side point to the two nephridia of that side (the two of the opposite side are not lettered)—each is seen to have an independent aperture.
xis the viscero-pericardial sac, the dotted line indicating its backward extension.
visc. per. apert, marks an arrow introduced into the right aperture of the viscero-pericardial sac.
r.e,r.e, point to the glandular enlarged walls of the afferent branchial vessels—two small glandular bodies of the kind are seen to project into each nephridial sac, whilst a larger body of the same kind depends from each of the four branchial afferent vessels into the viscero-pericardial sac.
v.c, Vena-cava.
vent, Ventricle of the heart.
ao, Cephalic aorta (the small abdominal aorta not drawn).
a.b.v, Branchial vessel.
e.v.b, Efferent branchial vessel.
Coelom, Blood-vascular System and Excretory Organs.—Nautilusand the other Cephalopoda conform to the general Molluscan characters in regard to these organs. Whilst the general visceral cavity forms a lacunar blood-system or series of narrow spaces, connected with the trunks of a well-developed vascular system, that part of the original coelom surrounding the heart and known as the Molluscan pericardium is shut off from this general blood-lymph system, and communicates, directly inNautilus, in the rest through the renal sacs, with the exterior. In the Cephalopoda this specialized pericardial cavity is particularly large, and has been recognized as distinct from the blood-carrying spaces, even by anatomists who have not considered the pericardial space of other Mollusca to be thus isolated. The enlarged pericardium, which may even take the form of a pair of sacs, has been variously named, but is best known as the viscero-pericardial sac or chamber. InNautilusthis sac occupies the whole of the postero-dorsal surface and a part of the antero-dorsal (see fig. 10,x), investing the genital and other viscera which lie below it, and having the ventricle of the heart suspended in it. Certain membranes forming incomplete septa, and a curious muscular band—the pallio-cardiac band—traverse the sac. The four branchial afferent veins, which in traversing the walls of the four renal sacs give off, as it were, glandular diverticula into those sacs, also give off at the same points four much larger glandular masses, which hang freely into the viscero-pericardial chamber (fig. 11,r.e). InNautilusthe viscero-pericardial sac opens to the exterior directly by a pair of apertures, one placed close to the right and one close to the left posterior renal aperture (fig. 5,visc. per). This direct opening of the pericardial sac to the exterior is an exception to what occurs in all other Mollusca. In all other Molluscs the pericardial sac opens into the renal organs, and through them or the one renal organ to the exterior. InNautilusthere is no opening from the viscero-pericardial sac into the renal sacs. Therefore the external pore of the viscero-pericardial sac may possibly be regarded as a shifting of the reno-pericardial orifice from the actual wall of the renal sac to a position alongside of its orifice. Parallel cases of such shifting are seen in the varying position of the orifice of the ink-bag in Dibranchiata, and in the orifice of the genital ducts of Mollusca, which in some few cases (e.g.Spondylus) open into the renal organs, whilst in other cases they open close by the side of the renal organs on the surface of the body. The viscero-pericardial sac of the Dibranchs is very large also, and extends into the dorsal region. It varies inshape—that is to say, in the extensions of its area right and left between the various viscera—in different genera, but in the Decapods is largest. In an extension of this chamber is placed the ovary ofSepia, whilst the ventricle of the heart and the branchial hearts and their appendages also lie in it. It is probable that water is drawn into this chamber through the renal sacs, since sand and other foreign matters are found in it. In all it opens into the pair of renal sacs by an orifice on the wall of each, not far from the external orifice (fig. 29,y, y′). There does not seem any room for doubting that each orifice corresponds to the reno-pericardial orifice which we have seen in the Gastropoda, and shall find again in the Lamellibranchia.
A, Part of the dorsal vascular trunk and transverse trunks of a worm.
B, Ventricle and auricles ofNautilus.
C, Of a Lamellibranch, ofChiton, or ofLoligo.
D, OfOctopus.
E, Of a Gastropod.
a, Auricle.
v, Ventricle.
ac, Arteria = cephalica = (aorta).
ai, Arteria abdominalis. The arrows show the direction of the blood-current.
The circulatory organs, blood-vessels and blood ofNautilusdo not differ greatly from those of Gastropoda. The ventricle of the heart is a four-cornered body, receiving a dilated branchial efferent vessel (auricle) at each corner (fig. 11). It gives off a cephalic aorta anteriorly, and a smaller abdominal aorta posteriorly. The diagram, fig. 12, serves to show how this simple form of heart is related to the dorsal vessel of a worm or of an Arthropod, and how by a simple flexure of the ventricle (D) and a subsequent suppression of one auricle, following on the suppression of one branchia, one may obtain the form of heart characteristic of the anisopleurous Gastropoda (excepting the Aspidobranchia). The flexed condition of the heart is seen inOctopus, and is to some extent approached byNautilus, the median vessels not presenting that perfect parallelism which is shown in the figure (B). The most remarkable feature presented by the heart ofNautilusis the possession of four instead of two auricles, a feature which is simply related to the metamerism of the branchiae. By the left side of the heart ofNautilus, attached to it by a membrane, and hanging loosely in the viscero-pericardial chamber, is the pyriform sac of Owen. This has been shown to be the rudimentary left oviduct or sperm-duct, as the case may be (E.R. Lankester and A.G. Bourne), the functional right ovi-sac and its duct being attached by a membrane to the opposite side of the heart.
The cephalic and abdominal aortae ofNautilusappear, after running to the anterior and posterior extremes of the animal respectively, to open into sinus-like spaces surrounding the viscera, muscular masses, &c. These spaces are not large, but confined and shallow. Capillaries are stated to occur in the integument. In the Dibranchs the arterial system is very much more complete; it appears in some cases to end in irregular lacunae or sinuses, in other cases in true capillaries which lead on into veins. An investigation of these capillaries in the light of modern histological knowledge is much needed. From the sinuses and capillaries the veins take origin, collecting into a large median trunk (the vena cava), which in the Dibranchs as well as inNautilushas a ventral (postero-ventral) position, and runs parallel to the long axis of the animal. InNautilusthis vena cava gives off at the level of the gills four branchial afferent veins (fig. 11,v.c.), which pass into the four gills without dilating. In the Dibranchs at a similar position the vena cava gives off a right and a left branchial afferent vein, each of which, traversing the wall of the corresponding renal sac and receiving additional factors, dilates at the base of the corresponding branchial plume, forming there a pulsating sac—the branchial heart. Attached to each branchial heart is a curious glandular body, which may possibly be related to the larger masses (fig. 11,r.e.) which depend into the viscero-pericardial cavity from the branchial afferent veins ofNautilus. From the dilated branchial heart the branchial afferent vessel proceeds, running up the adpallial face of the gill-plume. From each gill-plume the blood passes by the branchial efferent vessels to the heart, the two auricles being formed by the dilatation of these vessels.
The blood contains the usual amoeboid corpuscles, and a diffused colouring matter—the haemocyanin of Fredericque—which has been found also in the blood ofHelix, and in that of the ArthropodsHomarusandLimulus. It is colourless in the oxidized, blue in the deoxidized state, and contains copper as a chemical constituent.
The renal sacs and renal glandular tissue are closely connected with the branchial advehent vessels inNautilusand in the other Cephalopoda. The arrangement is such as to render the typical relations and form of a renal tube difficult to trace. In accordance with the metamerism ofNautilusalready noticed, there are two pairs of renal organs. Each assumes the form of a sac opening by a pore to the exterior. As is usual in renal tubes a glandular and a non-glandular portion are distinguished in each sac; these portions, however, are not successive parts of a tube, as happens in other cases, but they are localized areae of the wall of the sac. The glandular renal tissue is, in fact, confined to a tract extending along that part of the sac’s wall which immediately invests the great branchial afferent vein. The vein in this region gives off directly from its wall a complete herbage of little venules, which branch and anastomose with one another, and are clothed by the glandular epithelium of the renal sac. The secretion is accumulated in the sac and passed by its aperture to the exterior. Probably the nitrogenous excretory product is very rapidly discharged; inNautilusa pink-coloured powder is found accumulated in the renal sacs, consisting of calcium phosphate. The presence of this phosphatic calculus by no means proves that such was the sole excretion of the renal glandular tissue. InNautilusa glandular growth like that rising from the wall of the branchial vessel into its corresponding renal sac, but larger in size, depends from each branchial afferent vessel into the viscero-pericardial sac and forms the pericardial gland—probably identical with the “appendage” of the branchial hearts of Dibranchs.
The chief difference, other than that of number, between the renal organs of the Dibranchs and those ofNautilus, is the absence of the accessory growths depending into the viscero-pericardial space just mentioned, and, of more importance, the presence in the former of a pore leading from the renal sac into the viscero-pericardial sac (y,y′in fig. 29). The external orifices of the renal organs are also more prominent in Dibranchs than inNautilus, being raised on papillae (npin fig. 29;rin fig. 25). InSepiathe two renal sacs give off each a diverticulum dorsalwards, which unites with its fellow and forms a great median renal chamber, lying between the ventral portions of the renal organs and the viscero-pericardial chamber. InLoligothe fusion of the two renal organs to form one sac is still more obvious, since the ventral portions are united. InOctopusthe renal sacs are quite separate.
Gonads and Genital Ducts.—InNautilusit has been shown by E. Ray Lankester and A.G. Bourne that the genital ducts of both sexes are paired right and left, the left duct being rudimentary and forming the “pyriform appendage,” described by Sir R. Owen as adhering by membranous attachment to the ventricle of the heart, and shown by W. Keferstein to communicate by a pore with the exterior. The ovary (female gonad) or the testis (male gonad) lies inNautilus, as in the Dibranchs, in a distinct cavity walled off from the other viscera, near the centro-dorsal region. This chamber is formed by the coelomic or peritoneal wall; the space enclosed is originally part of the coelom, and inSepiaandLoligois, in the adult, part of the viscero-pericardial chamber. InOctopusit is this genital chamber which communicates by a right and a left canal with the renal sac, and is the only representative of pericardium. The ovary or testis is itself a growth from the inner wall of this chamber, which it only partly fills. InNautilusthe right genital duct, which is functional, is a simple continuation to the pore on the postero-dorsal surface of the membranous walls of the capsule in which lies the ovary or the testis, as the case may be. The gonad itself appears to represent a single median or bilateral organ.
The ovary forms a large projection into the genital coelom, and the coelomic epithelium is deeply invaginated into the mass of the gonad, so as to constitute an ovarian cavity communicating with the coelom by a narrow aperture. The ova originate in the epithelium, migrate below it and then, as they enlarge, project into the ovarian cavity, pushing the epithelium before them. Each ovum is surrounded by a follicular epithelium which is nourished by numerous blood-vessels, and which penetrates into the surface of the ovum in numerous folds. When mature, the ovum is contained in a membrane or chorion with a micropyle, and escapes by dehiscence of the follicle into the genital coelom and duct. In its passage to the exterior the ovum passes a glandular structure on the wall of the genital capsule, which probably secretes the gelatinous substance enclosing the eggs. In addition to this internal gland there are other accessory glands, which are not related to the genital duct or sac but are differentiations of the wall of the pallial cavity, and occur on the inner wall of the pallium inNautilus, on the somatic wall in Dibranchiata. InNautilusthey form a continuous mass. These produce the external envelopes of the eggs.
In the male the testis is a specialized portion of the wall of the genital coelom, and has a structure comparable to that of the ovary. The spermatozoa pass through an orifice from the cavity of the testis to the genital capsule, and thence to the spermiduct. The spermiduct is provided with a glandular pouch, and opens into a terminal reservoir known as Needham’s sac or the spermatophore sac. The function of this pouch is to form the spermatophore, which is an elastic tube formed of structureless secretion and invaginated into itself. The deeper part contains the spermatozoa, the external part is called the connective, and is usually much contracted and spirallycoiled. When the spermatophore is expelled into the water the connective is extended and evaginated, and the sac containing the sperms bursts. InNautilusthe spermatophore when uncoiled is a little over 30 mm. in length. These spermatophores are somewhat similar to those formed in certain pulmonate Gastropods.
The eggs are laid shortly after copulation. InNautilusthey are laid separately, each being about 4 cm. long and contained in two thick shells, the outer of which is partly open.
Fig.13.—Nervous system ofNautilus pompilius(from Genebaur, after Owen).
t, t, Ganglion-like enlargements on nerves passing from the pedal ganglion to the inner series of tentacles.
t′, Nerves to the tentacles of the outer or annular lobe.
b, Pedal ganglion-pair
a, Cerebral ganglion-pair.
c, Pleuro-visceral ganglionic band (fused pleural and visceral ganglion-pairs).
d, Genital ganglion placed on the course of the large visceral nerve, just before it gives off its branchial and its osphradial branches.
m, Nerves from the pleural ganglion to the mantle-skirt.
Nervous System.—Nautilus, like the other Cephalopoda, exhibits a great concentration of the typical Molluscan ganglia, as shown in fig. 13. The ganglia take on a band-like form, and are but little differentiated from their commissures and connectives—an archaic condition reminding us ofChiton. The special optic outgrowth of the cerebral ganglion, the optical ganglion (fig. 13,o), is characteristic. The cerebral ganglion-pair (a) lying above the oesophagus is connected with two suboesophageal ganglion-pairs, of band-like form. The anterior of these is the pedalb, b, and supplies the circumoral lobes and tentacles, and the funnel, a fact which proves the pedal origin of these organs. The hinder band is the visceral and pleural pair fused; from its pleural portion nerves pass to the mantle, from its visceral portion nerves to the branchiae and genital ganglion (fig. 13,d), and in immediate connexion with the latter is a nerve to the osphradium or olfactory papilla. A labial commissure arises by a double root from the cerebral ganglia and gives off a stomatogastric commissure, which passes under the pharynx immediately behind the radula and bears a buccal ganglion on either side.
Special Sense-Organs.—Nautiluspossesses a pair of osphradial papillae (fig. 4,olf) corresponding in position and innervation to Spengel’s organ placed at the base of the ctenidia (branchiae) in all classes of Mollusca. This organ has not been detected in other Cephalopoda.Nautiluspossesses other olfactory organs in the region of the head. Just below the eye is a small triangular process (not seen in our figures), having the structure of a shortened and highly-modified tentacle and sheath. By A. Valenciennes, who is followed by W. Keferstein, this is regarded as an olfactory organ. The large nerve which runs to this organ originates from the point of juncture of the pedal with the optic ganglion. The lamelliform organ upon the inner inferior tentacular lobe ofNautilusis possibly also olfactory in function. In Dibranchs behind the eye is a pit or open canal supplied by a nerve corresponding in origin to the olfactory nerve ofNautilusabove mentioned. Possibly the sense of taste resides in certain processes within the mouth ofNautilusand other Cephalopoda.
The otocysts ofNautiluswere discovered by J.D. Macdonald. Each lies at the side of the head, ventral to the eye, resting on the capito-pedal cartilage, and supported by the large auditory nerve which apparently arises from the pedal ganglion but originates in the cerebral. It has the form of a small sac, 1 to 2 mm. in diameter, and contains whetstone-shaped crystals, such as are known to form the otoliths of other Mollusca.
The eye ofNautilusis among the most interesting structures of that remarkable animal. No other animal which has the same bulk and general elaboration of organization has so simple an eye as that ofNautilus. When looked at from the surface no metallic lustre, no transparent coverings, are presented by it. It is simply a slightly projecting hemispherical box like a kettle-drum, half an inch in diameter, its surface looking like that of the surrounding integument, whilst in the middle of the drum-membrane is a minute hole (fig. 3,u). Sir R. Owen very naturally thought that some membrane had covered this hole in life, and had been ruptured in the specimen studied by him. It, however, appears from the researches of V. Hensen that the hole is a normal aperture leading into the globe of the eye, which is accordingly filled by sea-water during life. There is no dioptric apparatus inNautilus, and in place of refracting lens and cornea we have actually here an arrangement for forming an image on the principle of “the pin-hole camera.” There is no other eye known in the whole animal kingdom which is so constructed. The wall of the eye-globe is tough, and the cavity is lined solely by the naked retina, which is bathed by sea-water on one surface and receives the fibres of the optic nerve on the other (see fig. 14, A). As in other Cephalopods (e.g.fig. 33,Ri, Re, p), the retina consists of two layers of cells, separated by a layer of dark pigment. The most interesting consideration connected with this eye ofNautilusis found when the further facts are noted—(1) that the elaborate lens-bearing eyes of Dibranchiata pass through a stage of development in which they have the same structure as the eye ofNautilus—namely, are open sacs (fig. 34); and (2) that amongst other Mollusca examples of cephalic eyes can be found which in the adult condition are, like the eye ofNautilusand the developing eye of Dibranchs, simple pits of the integument, the cells of which are surrounded by pigment and connected with the filaments of an optic nerve. Such is the structure of the eye of the limpet (Patella), and in such a simple eye we obtain the clearest demonstration of the fact that the retina of the Molluscan cephalic eye, like that of the Arthropod cephalic eye and unlike that of the vertebrate myelonic eye, is essentially a modified area of the general epiderm, and that the sensitiveness of its cells to the action of light and their relation to nerve-filaments is only a specialization and intensifying of a property common to the whole epiderm of the surface of the body. What, however, strikes us as especially remarkable is that the simple form of a pit, which inPatellaserves to accumulate a secretion which acts as a refractive body, should inNautilusbe glorified and raised to the dignity of an efficient optical apparatus. In all other Mollusca, starting as we may suppose from the follicular or pit-like condition, the eye has proceeded to acquire the form of aclosedsac, the cavity of the closed vesicle being then filled partially or completely by a refractive body (lens) secreted by its walls (fig. 14, B). This is the condition attained in most Gastropoda. It presents a striking contrast to the simple Arthropod eye, where, in consequence of the existence of a dense exterior cuticle, the eye does not form a vesicle, and the lens is always part of that cuticle.
A,Nautilus(andPatella).
B, Gastropod (LimaxorHelix).
C, Dibranchiate Cephalopod (Oigopsid).
Pal, Eyelid (outermost fold).
Co, Cornea (second fold).
Ir, Iris (third fold).
Int1,2,3,4, Different parts of the integument.
l, Deep portion of the lens.
l1, Outer portion of the lens
Co.ep, Ciliary body.
R, Retina.
N.op, Optic nerve.
G.op, Optic ganglion.
x, Inner layer of the retina.
N.S., Nervous stratum of the retina. (From Balfour, after Grenacher.)
The development ofNautilusis still entirely unknown. Dr Arthur Willey, during his sojourn in the East Indies, made special efforts to obtain fertilized eggs, both by offering rewards to the native fishermen and collectors and by keeping the living adults in captivity, but without success.
Phylogeny and Classification.—AsNautilusis the only living genus of the Tetrabranchiata, our knowledge of all the rest is based upon the study of their fossil shells. A vast number of species of shell similar in structure to that ofNautilusare known, chiefly from Primary and Secondary formations. These are divided into two sub-orders by differences in the form and structure of the initial chamber. In the Nautiloidea this chamber has the form of an obtuse cone, on the apex of which is a slit-like mark or cicatrix, elongated dorso-ventrally and placed opposite to the blind end of the siphuncle, which indents the front wall of the initial chamber but does not enter its cavity. In the Ammonoidea, on the other hand, the initial chamber is inflated, and is spheroidal, oval or pyriform in shape, with no cicatrix, and separated from the first air-chamber by a constriction. The siphuncle also commences with a dilatation which deeply indents the front wall of the initial chamber, called the protoconch, but does not penetrate into its cavity. Munier-Chalmas has shown that the cavity of the protoconch is traversed by a tubular organ, the “prosiphon,” which does not communicate with the true siphuncle, the place of which it is supposed to take in the early life of the animal. It is generally held, as suggested byAlpheus Hyatt, that the initial chamber of the Nautiloidea corresponds not to the protoconch of the Ammonoids, but to the second chamber of the latter, and that there existed in the young Nautiloids a true initial chamber, a protoconch which was either uncalcified or deciduous. The shell of the living nautilus does not decide this question, as its early stages are unknown, and there is a little vacuity in the centre of the spirally coiled shell which may have been originally occupied by the true protoconch.
The septa in the Nautiloidea are generally concave towards the aperture of the shell, their curvature therefore directed backwards (fig. I); in the Ammonoidea, on the other hand, the convexity is usually towards the aperture, the curvature therefore directed forwards. The lines along which the edges of the septa are united to the shell are known as “sutures,” and these in the Nautiloidea are simply curved or slightly lobed, whereas in the Ammonoidea they are folded in various degrees of complexity; the projections of the suture towards the mouth of the shell are called saddles, those in the opposite direction lobes. The siphuncle in theNautiluspierces the centres of the septa, and in fossil Nautiloids it is usually central or sub-central. In a few cases it is marginal, and in that case may be external,i.e.ventral, or internal,i.e.dorsal. In Ammonoids the siphuncle is always marginal, and usually external. Its walls in the livingNautilusare strengthened by the deposit of calcareous granules, and in some fossil forms the wall is completely calcified. But this proper calcified wall is quite distinct from calcareous tubes surrounding the siphuncle, which are developed from the septa. In the pearly nautilus each septum is prolonged backwards at the point where it is pierced by the siphuncle, forming a shelly tube somewhat like the neck of a bottle. In many fossil forms these septal necks are continued from the septum from which they arise to the next, so that the siphuncle is enclosed in a complete secondary calcareous tube. In the majority of Nautiloids the septal necks are directed backwards, and they are said to be retrosiphonate. In the majority of the Ammonoids the septal necks are continued forwards from the septa to which they belong, and such forms are termed prosiphonate.
The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic periods. The Nautiloidea are the most ancient, appearing first in the Upper Cambrian, the genera being most numerous in the Palaeozoic period, and comparatively few surviving into the Secondary. On the other hand, the Ammonoidea are scarce in Palaeozoic formations, being represented in deposits earlier than the Carboniferous only by comparatively simple types, such asClymeniaandGoniatites. In the Secondary period Ammonoids were very abundant, both in genera and species and in individuals, and with few local exceptions none are known to have survived even to the commencement of the Tertiary. In the widest sense the genusNautilushas existed since the Ordovician (Silurian) period, but the oldest types are not properly to be placed in the same genus as the existing form. Even with this qualification the genus is very ancient, shells very similar to those of the livingNautilusbeing found in the Upper Cretaceous.
It has been maintained by some zoologists that the Ammonoidea were Dibranchiate, though it would not follow from this that the shell was, therefore, internal. They are, however, generally classed with the Tetrabranchiata, and the absence of all evidence of the possession of an ink-sac is in favour of this view. There can be little doubt that they gave rise to the Dibranchiata.
About 2500 fossil species are included in the Nautiloidea, but only a few species of the genusNautilussurvive. Some of the fossil forms are very large, the shell reaching a length of 2 metres, or 6 ft. 6 in. Of the Ammonoidea more than 5000 species have been described, and some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in diameter.
Associated with various forms of Ammonoids there have been found peculiar horny or calcified plates, sometimes contained within the body-chamber of the shell, sometimes wholly detached. The most typical form of these structures has been namedaptychus.It consists of two bilaterally symmetrical halves, of somewhat semicircular shape, and attached to one another by their straight inner margins, like a pair of doors. In some cases the aptychus is thin and horny, but more often it is thick and calcified, in which case the principal layer has a peculiar cellular structure. The surface may be smooth or sculptured, and one side is usually marked by concentric lines of growth. Another type is similar, except that the two halves are united in the middle line; bodies of this character are calledsynaptychus; they occur in the body-chamber of species ofScaphites. Another form calledanaptychusconsists of a thin horny undivided plate which is concentrically striated. This is associated with species ofAmmonitesandGoniatites.
Many theories have been proposed in explanation of these structures. According to Sir Richard Owen, the aptychus is an operculum developed in a part of the body corresponding to the hood ofNautilus. E. Ray Lankester suggested that the double plate was borne on the surface of the nidamental gland, with the form and sculpturing of which inNautilusit closely agrees. On this view the aptychus would occur only in females. The most recent view is that these structures could not have been opercula because of their constant position inside the body-chamber, and that they were not external secretions at all, but a calcified internal cartilage situated at the base of the funnel.
Classification of Tetrabranchiata.—Cephalopoda in which the mantle is entirely enclosed by a multilocular siphunculated shell, which may or may not be coiled. Only the last compartment of the shell occupied by the body of the animal. Numerous pedal tentacles around the mouth, which are retractile within sheaths. Halves of the funnel not united. Two pairs of ctenidia, and two pairs of renal tubes without reno-pericardial apertures. Pericardium opens directly to exterior. Cephalic cartilage wholly ventral. Optic vesicles with apertures, without crystalline lens.
Sub-order1.Nautiloidea.—Initial chamber not inflated, with dorso-ventral cicatrix at extremity.
Fam. 1.Orthoceratidae. Shell straight or slightly curved, with a simple aperture, large terminal chamber and cylindrical siphuncle.Orthoceras, Silurian to Trias.Baltoceras, Silurian.Fam. 2.Actinoceratidae. Shell straight or slightly curved, with wide siphuncle contracted at level of septa.Actinoceras, Silurian to Carboniferous.Discosorus, Silurian.Huronia, Silurian.Loxoceras, Silurian to Carboniferous.Fam. 3.Endoceratidae. Shell straight, with wide margina siphuncle, necks produced into tubes fitting into one another.Endoceras, Silurian.Fam. 4.Gomphoceratidae. Shell globular, straight or arcuate, aperture contracted.Gomphoceras, Silurian.Phragmoceras, Silurian.Fam. 5.Ascoceratidae. Shell straight, ampulliform, summit truncate, terminal chamber extending nearly whole length of shell ventrally.Ascoceras, Silurian.Glossoceras, Silurian.Fam. 6.Poterioceratidae. Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa.Poterioceras, Silurian to Carboniferous.Streptoceras, Silurian.Fam. 7.Cyrtoceratidae. Shell slightly curved, aperture simple, siphuncle wide, septa approximated.Cyrtoceras, Devonian.Fam. 8.Lituitidae. Shell coiled in one plane with the terminal part uncoiled, aperture contracted.Lituites, Silurian.Ophidioceras, Silurian.Fam. 9.Trochoceratidae. Shell helicoidally coiled, dextral or sinistral, the last whorl generally uncoiled.Trochoceras, Devonian.Adelphoceras, Devonian.Fam. 10.Nautilidae. Shell coiled in one plane, aperture wide and simple, siphuncle central.Nautilus, recent.Trocholites, Silurian.Gyroceras, Silurian to Carboniferous.Hercoceras, Silurian.Ptenoceras, Devonian.Discites, Carboniferous.Fam. 11.Bactritidae. Shell straight, conical, siphuncle narrow and marginal, necks long, infundibuliform, sutures undulating.Bactrites, Silurian and Devonian.
Fam. 1.Orthoceratidae. Shell straight or slightly curved, with a simple aperture, large terminal chamber and cylindrical siphuncle.Orthoceras, Silurian to Trias.Baltoceras, Silurian.
Fam. 2.Actinoceratidae. Shell straight or slightly curved, with wide siphuncle contracted at level of septa.Actinoceras, Silurian to Carboniferous.Discosorus, Silurian.Huronia, Silurian.Loxoceras, Silurian to Carboniferous.
Fam. 3.Endoceratidae. Shell straight, with wide margina siphuncle, necks produced into tubes fitting into one another.Endoceras, Silurian.
Fam. 4.Gomphoceratidae. Shell globular, straight or arcuate, aperture contracted.Gomphoceras, Silurian.Phragmoceras, Silurian.
Fam. 5.Ascoceratidae. Shell straight, ampulliform, summit truncate, terminal chamber extending nearly whole length of shell ventrally.Ascoceras, Silurian.Glossoceras, Silurian.
Fam. 6.Poterioceratidae. Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa.Poterioceras, Silurian to Carboniferous.Streptoceras, Silurian.
Fam. 7.Cyrtoceratidae. Shell slightly curved, aperture simple, siphuncle wide, septa approximated.Cyrtoceras, Devonian.
Fam. 8.Lituitidae. Shell coiled in one plane with the terminal part uncoiled, aperture contracted.Lituites, Silurian.Ophidioceras, Silurian.
Fam. 9.Trochoceratidae. Shell helicoidally coiled, dextral or sinistral, the last whorl generally uncoiled.Trochoceras, Devonian.Adelphoceras, Devonian.
Fam. 10.Nautilidae. Shell coiled in one plane, aperture wide and simple, siphuncle central.Nautilus, recent.Trocholites, Silurian.Gyroceras, Silurian to Carboniferous.Hercoceras, Silurian.Ptenoceras, Devonian.Discites, Carboniferous.
Fam. 11.Bactritidae. Shell straight, conical, siphuncle narrow and marginal, necks long, infundibuliform, sutures undulating.Bactrites, Silurian and Devonian.
Sub-order2.Ammonitoidea,—Initial chamber spheroidal; siphuncle narrow and simple; septa convex towards aperture; sutures complex.
Tribe1.Retrosiphonata.—Siphuncular necks projecting behind the septa as in Nautiloidea. Sutures form simple undulations. Occur exclusively in Palaeozoic strata from Devonian upwards.
Fam. 1.Goniatitidae. Shell nautiloid, with simple sutures and ventral siphuncle.Goniatites, Devonian and Carboniferous.Anarcestes, Devonian.Fam. 2.Clymeniidae. Shell nautiloid, with simple sutures, siphuncle dorsal, that is, internal.Clymenia, Upper Devonian.
Fam. 1.Goniatitidae. Shell nautiloid, with simple sutures and ventral siphuncle.Goniatites, Devonian and Carboniferous.Anarcestes, Devonian.
Fam. 2.Clymeniidae. Shell nautiloid, with simple sutures, siphuncle dorsal, that is, internal.Clymenia, Upper Devonian.
Tribe2.Prosiphonata.—Siphuncular necks projecting in front of the septa. Sutures form deeply indented lobes and saddles.
Fam. 1.Arcestidae. Globular and smooth or nearly smooth, with reduced umbilicus, terminal chamber very deep, an aptychus present.Popanoceras, Permian.Cyclolobus, Permian,Arcestes, Trias.Lobites, Trias.Fam. 2.Tropitidae. Shells globular, but having radiating and tuberculated costae.Thalassoceras, Permian.Tropites, Trias.Sibirites, Trias.Fam. 3.Ceratitidae. Shells coiled, with a large umbilicus, terminal chamber short, sutures with simple saddles.Trachyceras, Upper Trias.Ceratites, Trias.Dinarites, Trias.
Fam. 1.Arcestidae. Globular and smooth or nearly smooth, with reduced umbilicus, terminal chamber very deep, an aptychus present.Popanoceras, Permian.Cyclolobus, Permian,Arcestes, Trias.Lobites, Trias.
Fam. 2.Tropitidae. Shells globular, but having radiating and tuberculated costae.Thalassoceras, Permian.Tropites, Trias.Sibirites, Trias.
Fam. 3.Ceratitidae. Shells coiled, with a large umbilicus, terminal chamber short, sutures with simple saddles.Trachyceras, Upper Trias.Ceratites, Trias.Dinarites, Trias.
Some genera with helicoidal shells are related to these coiled forms, viz.Cochloceras, Trias; also some straight forms,e.g. Rhab-doccras, Trias.
Fam. 4.Pinacoceratidae. Shell compressed, smooth, terminal chamber short, sutures very complicated, convex.Pinacoceras, Trias.Fam. 5.Phylloceratidae. Shell coiled, the whorls overlapping each other, sutures formed of numerous lobes and saddles.Phytloceras, Jurassic.Fam. 6.Lytoceratidae. Shell discoid, whorls loosely united or uncoiled, sutures deeply indented, but with only three saddles and lobes.Lytoceras, Jurassic and Cretaceous.Macroscaphites, Cretaceous.Reunites, Cretaceous.Ptychoeeras, Cretaceous.Turrilites, Cretaceous.Baculites, Cretaceous.Fam. 7.Ammonitidae. Shell coiled, with narrow whorls which do not embrace one another, aperture simple, a horny anaptychus present.Ammonites, Jurassic.Arietites, Jurassic.Aegoceras, Lias.Fam. 8.Harpoceratidae. Shell discord and flattened, with a carinated border, aperture provided with lateral projections,a calcareous aptychus, formed of two pieces.Harpoceras, Jurassic.Oppelia, Jurassic.Lissoceras, Jurassic and Cretaceous.Fam. 9.Amaltheidae. Shell flattened, with a prominent carina continued anteriorly into a rostrum.Amaltheus, Lias.Cardioceras, Jurassic.Schloenbachia, Cretaceous.Fam. 10.Stephanoceratidae. Shell not carinated, but with radiating costae, which are often bifurcated, aperture often with lateral projections which contract it, aptychus formed of two pieces.Stephanoceras, Morphoceras, Pensphinctes, Peltoceras, Jurassic.Hoplites, Cretaceous.Acanthoceras, Cretaceous.Cosmoceras, Jurassic. Various more or less uncoiled forms are related to this family, viz.Scaphites, Crioceras, Cretaceous.
Fam. 4.Pinacoceratidae. Shell compressed, smooth, terminal chamber short, sutures very complicated, convex.Pinacoceras, Trias.
Fam. 5.Phylloceratidae. Shell coiled, the whorls overlapping each other, sutures formed of numerous lobes and saddles.Phytloceras, Jurassic.
Fam. 6.Lytoceratidae. Shell discoid, whorls loosely united or uncoiled, sutures deeply indented, but with only three saddles and lobes.Lytoceras, Jurassic and Cretaceous.Macroscaphites, Cretaceous.Reunites, Cretaceous.Ptychoeeras, Cretaceous.Turrilites, Cretaceous.Baculites, Cretaceous.
Fam. 7.Ammonitidae. Shell coiled, with narrow whorls which do not embrace one another, aperture simple, a horny anaptychus present.Ammonites, Jurassic.Arietites, Jurassic.Aegoceras, Lias.
Fam. 8.Harpoceratidae. Shell discord and flattened, with a carinated border, aperture provided with lateral projections,a calcareous aptychus, formed of two pieces.Harpoceras, Jurassic.Oppelia, Jurassic.Lissoceras, Jurassic and Cretaceous.
Fam. 9.Amaltheidae. Shell flattened, with a prominent carina continued anteriorly into a rostrum.Amaltheus, Lias.Cardioceras, Jurassic.Schloenbachia, Cretaceous.
Fam. 10.Stephanoceratidae. Shell not carinated, but with radiating costae, which are often bifurcated, aperture often with lateral projections which contract it, aptychus formed of two pieces.Stephanoceras, Morphoceras, Pensphinctes, Peltoceras, Jurassic.Hoplites, Cretaceous.Acanthoceras, Cretaceous.Cosmoceras, Jurassic. Various more or less uncoiled forms are related to this family, viz.Scaphites, Crioceras, Cretaceous.