Order 2. Dibranchiata(= Holosiphona, Acetabulifera)
Characters.—Cephalopoda in which the inflected margins of the epipodia are fused so as to form a complete tubular siphon (fig. 24,i). The circumoral lobes of the fore-foot carry suckers disposed upon them in rows,nottentacles (see figs. 15, 24). There is a single pair of typical ctenidia (fig. 25) acting as gills (hence Dibranchiata), and a single pair of renal organs, opening by apertures right and left of the median anus (fig. 25,r) and by similar internal pores into the pericardial chamber, which consequently does not open directly to the surface as inNautilus. The oviducts are sometimes paired right and left (Octopoda, Oigopsida), sometimes that of one side only is developed (Myopsida). The sperm-duct is always single except, according to W. Keferstein, inEledone moschata.
A plate-like shell is developed in a closed sac formed by the mantle (figs 20, 21), except in the Octopoda, which have none, and inSpirula(fig. 17, D) and the extinctBelemnitidae, &c., which have a small chambered shell resembling that ofNautiluswith or without the addition of plate-like and cylindrical accessory developments (fig. 17, A, C, fig. 19).
The pair of cephalic eyes are highly-developed vesicles with a refractive lens (fig. 33), cornea and lid-folds,—the vesicle being in the embryo, an open sac like that ofNautilus(fig. 34). Osphradia are not present, but cephalic olfactory organs are recognized. One or two pairs of large salivary glands with long ducts are present. An ink-sac formed as a diverticulum of the rectum and opening near the anus is present in all Dibranchiata (fig. 25,t), and has been detected even in the fossilBelemnitidae.Branchial hearts are developed on the two branchial afferent blood-vessels (fig. 28,vc′,vi).
A,Cheiroteuthis Veranyi, d’Orb. (from the Mediterranean).
B,Thysanoteuthis rhombus, Troschel (from Messina).
C,Loligopsis cyclura, Fér. and d’Orb. (from the Atlantic Ocean).
A,Conoteutliis dupiniana, d’Orb. (from the Neocomian of France).
B, ShellSepia orbigniana. Fér. (Mediterranean).
C, Shell ofSpirulirostra Bellardii, d’Orb. (from the Miocene of Turin). The specimen is cut so as to show in section the chambered shell and the laminated “guard” deposited upon its surface.
D, Shell ofSplrula laevis, Gray (New Zealand).
In the Dibranchiata the shell shows various stages of degeneration, culminating in its complete disappearance inOctopus. As in other Mollusca, there is a tendency in Cephalopods for the mantle to extend over the outside of the shell from its edges, and when these secondary mantle-folds entirely cover the shell and meet or fuse together the shell is surrounded by the mantle both externally and internally, and is said to be internal, though it remains always a cuticular structure external to the epidermis. This procebs is generally accompanied by a reduction of the size of the shell in comparison with that of the body, so that the relations of the two are gradually reversed, the body outgrows its house and instead of themantle being enclosed by the shell, the shell is enclosed by the mantle. The earliest stage of this process is shown in the recentSpirula,though it is perhaps not impossible that in some of the later fossil Ammonoids the shell was becoming more and more internal. The shell ofSpirula(fig. 18) is coiled somewhat like that ofNautilus, but the coils are not in contact, the direction of the coil is endogastric or ventral instead of exogastric, and the shell is very much smaller than the body. Like that ofNautilusit is divided by septa and traversed by a siphuncle. The relation of the animal to the terminal chamber is as inNautilus,but the body extends far beyond the aperture, and folds of the mantle grow up over the shell and cover it everywhere except part of the dorsal and ventral surfaces.
A, Dorsal aspect.pa, Mantle.B, Ventral aspect.po, Posterior fossa.a, Arms.sh, Shell.e, Eyes.te, Tentacular arms.fi, Fins.td, Terminal pallial disk.fu, Funnel.
The next modification in the enclosed shell is the addition to it of secondary deposits of calcareous matter, by the inner surface of the shell-sac. Successive layers are deposited on the posterior part of the original shell, whether coiled or straight, and these layers form a conical mass, which may attain great thickness. A somewhat coiled shell with such a deposit is seen inSpirulirostra(fig. 17, C) of the Miocene. In the next stage of modification secondary secretion forms a long and broad projection of the dorsal lip of the aperture; this is well developed in the belemnites (fig. 19). Thus in these modified shells three parts are to be distinguished: the original septate shell, which has been called the phragmacone; the posterior conical deposit, called the rostrum or guard; and the anterior somewhat flat projection, called the proostracum. In the living Dibranchiata other thanSpirulathe phragmacone and rostrum have become very rudimentary. The shell ofSepia(fig. 20) consists almost entirely of the proostracum, the little ventral hollow posteriorly representing the phragmacone, and the posterior pointed projection, the rostrum. In theOigopsidathe shell is represented by a proostracum which is no longer calcified by forms a chitinous plume or gladuius, and a similar rudiment occurs inLoliginidac(fig. 21) andSepiolidae. Lastly, in the Octopoda the shell is represented only by small chitinous rudiments to which the retractor muscles of the head and funnel are attached; these are paired inOctopus, unpaired in other cases as inCirrhoteuthis.
The early appearance of the sac of the mantle in which the shell is enclosed has led to an erroneous identification of this sac with the primitive shell-sac or shell-gland of the Molluscan embryo. The first appearance of the shell-sac in Dibranchiata is shown in figs. 35, 36. Its formation as an open upgrowth of the centro-dorsal area, and the fact that it appears and disappears without closing inArgonautaandOctopus, was demonstrated by E. Ray Lankester.
InArgonauta(the paper nautilus) the female only possesses a shell, in which the body is contained; but this is nothomologous with the true shell in other cases; it is a structuresui generissecreted by the expanded arms of the dorsal pair which are closely applied to it on either side (fig. 22).
a, Neck.
b, Eye.
c, The eight short arms.
d, Long prehensile arms, the clavate extremities of which are provided with suckers at e, and with a double row of hooks beyond at f. The temporary conjunction of the arms by means of the suckers enables them to act in combination.
t1, t2, t3, t4, The first, second, third and fourth arms or processes of the fore-foot.
h, The third arm of the right side hectocotylized.
x, The apical sac of the hectocotylized arm.
y, The filament which issues from the sac when development is complete.
i, The siphon.
Head, Foot, Mantle and Mantle-cavity.—If we now compare the fore-foot of the Dibranchiata with that ofNautilus, we find in the first place a more simple arrangement of its lobes, which are either four or five pairs of tapering processes (called “arms”), arranged in a series around the buccal cone, and a substitution of suckers for tentacles on the surface of these lobes (figs. 15 and 24). The most dorsally placed pair of arms, corresponding to the two sides of the hood ofNautilus, are in reality the most anterior, and are termed the first pair. In the Octopoda there are four pairs of these arms (fig. 38), in the Decapoda five pairs, of which the fourth is greatly elongated (figs. 15, 16). InSepia,SepiolaandRossia, each of these long arms is withdrawn into a pouch beside the head, and is only ejected for the purpose of prehension. InLoligothey are completely retractile, very slightly so in the majority of the Oigopsida, and inRhynchoteuthisthey are united to form a beak-like appendage. A gradual reduction of the tentacular arms can be seen in the Decapoda, leading to their total absence in Octopoda; thus inLeachia,Chaunoteuthisand others these arms are reduced to mere stumps. In someCheiroteuthidaeandCranchiidaethe ordinary or sessile arms, especially the dorsal pairs, are reduced. In the Octopoda they are not unfrequently connected by a web, and form an efficient swimming-bell,e.g.inCirrhoteuthidaeandAmphuretidae. The suckers are placed on the adoral surface of the arms, and may be in one, two or four rows, and very numerous. In place of suckers in some genera,e.g.Veranya, we find on certain arms or parts of the arms horny hooks; in other cases a hook rises from the centre of each sucker. The hooks on the long arms ofOnychoteuthisare drawn in fig. 23. In various species ofCheiroteuthisthe suckers on the tentacular arms are very feeble, but the bottom of the cup is covered by a number of anastomosed epithelial filaments which are used as a fishing-net. The fore-foot, with its apparatus of suckers and hooks, is in the Dibranchiata essentially a prehensile apparatus, though the whole series of arms in the Octopoda serve as swimming organs, and in many (e.g.the common octopus or poulp) the sucker-bearing surface is used as a crawling organ.Fig.25.—View of the postero-ventral surface of a maleSepia, obtained by cutting longitudinally the firm mantle-skirt and drawing the divided halves apart. This figure is strictly comparable with fig. 4. (From Gegenbaur.)C, The head.J, The mid-foot or siphon, which has been cut open so as to display the valvei.R, The glandular tissue of the left nephridium or renal-sac, which has been cut open (see fig. 29).P, P, The lateral fins of the mantle-skirt.Br, The single pair of branchiae (ctenidia).a, The anus—immediately below it is the opening of the ink-bag.c, Cartilaginous socket in the siphon to receivec′, the cartilaginous knob of the mantle-skirt—the two constituting the “pallial hinge apparatus” characteristic of Decapoda, not found in Octopoda.g, The azygos genital papilla and aperture.′i, Valve of the siphon (possibly the rudimentary hind-foot)m, Muscular band connected with the fore-foot and mid-foot (siphon) and identical with the muscular masskin fig. 3.r, Renal papillae, carrying the apertures of the nephridia.v.br, Branchial efferent blood-vessel.v br′, Bulbous enlargements of the branchial blood-vessels (see figs 28, 29).t, Ink-bag.In the males of the Dibranchiata one of the arms is more or less modified in connexion with the reproductive function, and is called the “hectocotylized arm.” This name is derived from the condition assumed by the arm in those cases in which its modification is carried out to the greatest extent. These cases are those of the OctopodsArgonauta argoandOcythoe catenulata(fig. 24). In the males of these the third arm (on the left side inArgonauta, on the right side inOcythoe) is found before the breeding season to be represented by a globular sac of integument. This sac bursts, and from it issues an arm larger than its neighbours, having a small sac at its extremity inOcythoe(fig. 24.x), from which subsequently a long filament issues. Before copulation the male charges this arm with the spermatophores or packets of spermatozoa removed from its generative orifice beneath the mantle-skirt, and during coitus the arm becomes detached and is left adhering to the female by means of its suckers. A new arm is formed at the cicatrix before the next breeding season. The female, being much larger than the male, swims away with the detached arm lodged beneath her mantle-skirt. There, in a way which is not understood, the fertilization of the eggsis effected. Specimens of the femaleOcythoëwith the detached arm adherent were examined by Cuvier, who mistook the arm for a parasitic worm and gave to it the nameHectocotylus. Accordingly, the correspondingly modified arms of other Cephalopoda are said to be hectocotylized. J.J.S. Steenstrup has determined the hectocotylized condition of one or other of the arms in a number of male Dibranchs as follows:—in all, exceptingArgonautaandOcythoeandTremoctopus, the modification of the arm is slight, consisting in a small enlargement of part or the whole of the arm, and the obliteration of some of its suckers; inOctopusandEledonethe third right arm is hectocotylized; inRossiaandSepiolathe fourth left arm is hectocotylized along its whole length, and the fourth right arm also in the middle only; inSepiathe fourth left arm is modified at its base only; inSepioteuthis, the same at its apex; inLoligo, the same also at its apex; inLoliolus, the same along its whole length; inOmmatostrephes,OnychoteuthisandLoligopsisno hectocotylized arm has hitherto been observed. Thus, speaking generally, it is one or both of the fourth pair of short arms which are modified in the Decapoda, of the third pair in the Octopoda. In the pallial cavity are situated one pair of gills in the Dibranchiata (fig. 25), attached dorsally along the whole of their afferent borders. On each side of the branchia is a series of lamellae, least in number in the Octopoda. Each lamella is transversely folded, and the folds are in turn folded, so that the respiratory surface is increased. On the somatic wall of the pallial cavity, between and ventral to the gills, are the following apertures: the anus and opening of the ink-sac, close together in the median line; a pair of apertures of the renal sacs, on either side of the median line; external to the renal orifice, on the left side, the genital aperture inCirrhoteuthidaeand Myopsida. In other Octopoda, and in nearly all the Oigopsida among the Decapoda, the genital ducts are paired in the female, but only the left is developed in the male. The funnel forms a complete tube in the Dibranchiata, and in the majority of the Decapoda, as inNautilus, it is provided with an internal valve projecting from its somatic surface, which allows water to pass outwards but prevents it passing inwards. The mantle performs rhythmical respiratory movements of expansion and contraction, the water entering between funnel and mantle and passing out through the funnel. In Decapoda the edge of the mantle bears internally on each side a cartilaginous projection which fits into a corresponding depression on the external surface of the funnel; this is called the “resisting apparatus,” and serves to make the union of mantle and funnel firmer during expiration. More powerful expiratory movements are used for sudden retrograde locomotion through the water.Fig.26.—Diagram representing a vertical approximately median antero-posterior section ofSepia officinalis(from a drawing by A.G. Bourne). The lettering corresponds with that of fig. 10, with which this drawing is intended to be compared.a, Shell (here enclosed by a growth of the mantle).b, The nuchal plate (here a cartilage).c, (The reference line should be continued through the black area representing the shell to the outline below it), the integument covering the visceral hump.d, The reflected portion of the mantle-skirt forming the sac which encloses the shell.e, The inferior margin of the mantle-skirt (mouth of the pallial chamber).f, The pallial chamber.g, The vertically cut median portion of the siphon.i, The valve of the siphon.m, The two upper lobes of the fore-foot.n, The long prehensile arms of the same.o, The fifth or lowermost lobe of the fore-foot.p, The third lobe of the fore-foot.q, The buccal membrane.v, The upper beak or jaw.s, The lower beak or jaw.t, The lingual ribbon.x, The viscero-pericardial sac.n.c, The nerve-collar.cr, The crop.gizz, The gizzard.an, The anus.c.t, The left ctenidium or gill-plume.vent, Ventricle of the heart.a.b.v, Afferent branchial vessel.e.b.v, Efferent branchial vessel.re, Renal glandular mass.n.n.a, Left nephridial aperture.visc.per.apert, Viscero-pericardial aperture (see fig. 29).br.b, Branchial heart.app, Appendage of the same.i.s, Ink-bag.Luminous Organs.—In certain Oigopsida living in deep water,e.g.Histioteuthis, Calliteuthis, Histiopsis, Pterygioteuthis, the surface of the skin bears photogenous organs directed towards the oral extremity. Anatomically these consist of a deeper photogenous layer and a more superficial refracting layer. In some cases,e.g.Pterygioteuthis, they occur even within the mantle-cavity.Fins.—In the majority of the Decapoda and in theCirrhoteuthidae, the mantle is produced into lateral symmetrical expansions which have the function of fins. They originate at the aboral extremity where they remain inSpirula(fig. 18). In most other Oigopsida they are terminal, but more dorsal than ventral,e.g.Loligopsis(fig. 16), and there may be two on each side, as inGrimalditeuthis. In other cases they extend laterally along a greater length of the body, as inSepia(fig. 15). InCtenopteryxthey have a superficial resemblance to the fins of fishes, consisting of a thin membrane supported by a series of muscular rods.Fig.27.—Alimentary canal ofLoligo sagittata(from Gegenbaur). The buccal mass is omitted.oe, Oesophagus.v, The stomach opened longitudinally.x, Probe passed through the pylorus.c, Commencement of the caecum.e, Its spiral portion.i, Intestine.a, Ink-bag.b, Its opening into the rectum.Chromatophores.—These are characteristic of the Dibranchiata, apparently absent inNautilus. They are originally single cells of ectodermic origin which sink below the epidermis and become connected with radiating muscular fibres. The cells are single but multinuclear. Different cells contain pigments of different colours, yellow, brown, red or blue. Each cell in life is in constant tremulous movement; under the influence of nervous excitement the cells are suddenly expanded or contracted, producing blushes of colour and pallor. By reflex action of which the afferent stimulus acts upon the eyes as in fishes, the chromatophores assume a condition which approximates the colour of the animal to that of surrounding objects. In the Decapoda there are also reflecting elements which produce iridescent hues.Aquiferous Cavities.—In addition to the pockets into which the tentacular arms of Decapoda are retracted, there are in several Dibranchiata cavities in the integument which open to the exterior by special pores but have no communication with the vascular system or other internal cavities of the body. InOcythoethere are such pores on the back of the head and at the base of the funnel; buccal pouches on the ventral side of the mouth, internal to the arms, occur in some genera, one inLoligo, two inSepia. In some species ofSepiathere are pouches in the mantle.Alimentary Tube.—The principal differences fromNautilusare the following:—the mandibles are similar in shape, but are chitinous, not calcified. In the radula there are three teeth on each side of the median tooth in each row, except inGonatus, in which there are only two lateral teeth, and theCirrhoteuthidae, in which the radula has entirely disappeared. In front of the radula is the so-called tongue, a fleshy projection corresponding to the sub-radular organ of other Mollusca.In most of the Dibranchiata there are two pairs of salivary glands. In the Decapoda the ducts of the posterior pair unite into a median duct which opens on the surface of the sub-radular organ. The anterior pair is but slightly developed except in the Oigopsida. In the Octopoda there are also two pairs, but the posterior pair, except inCirrhoteuthiswhere they are absent, are large and displaced backwards, being situated near the oesophageal proventriculus. Connected with the intestine immediately beyond the pylorus is a thin-walled caecum, spherical inRossiaandLeachia, elongated inLoligo, but usually coiled into a spiral (fig. 27). The hepatic ducts open into the caecum. The liver is developed as a paired gland, more or less fused into one in the adult, but the ducts are always paired. The ducts are covered by a number of glandular follicles forming what is called the pancreas.The ink-sac, absent inNautilus, is a rectal caecum developed from its dorsal wall. It is present in all Dibranchiata exceptOctopus arcticus, O. piscatorumandCirrhoteuthis. It consists of a deeper part or gland proper and a reservoir. It extends to the posterior extremity of the body inSepia, but inOctopodais usually embedded in the surface of the liver. The pigment of the secretion is melanin, and its function is to produce a dense opacity in the water, which conceals the animal.Vascular System(fig. 28).—The ventricle lies in the pericardial cavity, except in Octopoda where this cavity is much reduced. The auricles, one pair, are contractile expansions of the efferent branchial vessels. The heart gives off an anterior or cephalic and a posterior or abdominal aorta. The vascular system is almost perfect, arteries and veins being united by capillaries. The principal vein is a venacava passing backwards ventrally from the cephalic region and dividing into two afferent branchial veins, each of which receives a pallial and an abdominal vein. Each of these afferent branchial vessels is enclosed in the cavity of a renal organ and is covered externally by the glandular tissue which forms the excretory part of the “kidney” (fig. 29). Each afferent vessel is expanded into a contractile branchial heart, which is provided with a glandular appendage. The latter corresponds to the glandular masses which are attached to the afferent branchial veins inNautilus, and to the pericardial glands of other Molluscs.Fig.28.—Circulatory and excretory organs ofSepia(from Gegenbaur, after John Hunter).br, Branchiae (ctenidia).c, Ventricle of the heart.a, Anterior artery (aorta).a′, Posterior artery.v, The right and left auricles (enlargements of the efferent branchial veins).v′, Efferent branchial vein on the free face of the gill-plume.v.c, Vena cava.vi, vc′, Afferent branchial vessels (branches of the vena cava, see fig. 29).vc″, Abdominal veins.x, Branchial hearts and appendages.re, e, Glandular substance of the nephridia developed on the wall of the great veins on their way to the gills. The arrows indicate the direction of the blood-current.Fig.29.—Diagram of the nephridial sacs, and the veins which run through them, inSepia officinalis(after Vigelius). The nephridial sacs are supposed to have their upper walls removed.v.c, Vena cava.r.d.v.c, Right descending branch of the same.r.s.v.c, Left descending branch of the same.v.b.a, Vein from the ink-bag.v.m, Mesenteric vein.v.g, Genital vein.v.a.d, Right abdominal vein.v.a.s, Left abdominal vein.v.p.d, Right pallial vein.v.p.s, Left pallial vein.c.b, Branchial heart.x, Appendage of the same.c.v, Capsule of the branchial heart.np, External aperture of the right nephridial sac.y, Reno-pericardial orifice placing the left renal sac or nephridium in communication with the viscero-pericardial sac, the course of which below the nephridial sac is indicated by dotted lines.y′, The similar orifice of the right side.a.r, Glandular renal outgrowths.w.k, Viscero-pericardial sac (dotted outline).Fig.30.Fig.31.Figs. 30, 31.—Nerve-centres ofOctopus. Figure 30 gives a view from the dorsal aspect, figure 31 one from the ventral aspect.buc, The buccal mass.ped, Pedal ganglion.opt, Optic ganglion.cer, Cerebral ganglion.pl, Pleural ganglion.visc, Visceral ganglion.oes, Oesophagus.f, Foramen in the nerve-mass formed by pedal, pleural and visceral ganglion-pairs, traversed by a blood-vessel.Fig.32.—Lateral view of the nervous centres and nerves of the right side ofOctopus vulgaris(from a drawing by A.G. Bourne).bg, Buccal ganglion.cer, Cerebral ganglion.ped, Pedal ganglion.pl, Pleural, and visc., visceral region of the pleuro-visceral ganglion.gang. stell, The right stellate ganglion of the mantle connected by a nerve to the pleural portion.n.visc, The right visceral nerve.n.olf, Its (probably) olfactory branches.n.br, Its branchial branches.Coelom.—The coelom forms a large sac with a constriction between the anterior or pericardial division and the posterior or genital division, and it is produced into lateral diverticula which contain the branchial hearts; but in the Octopoda the pericardial division is suppressed and the genital division communicates by long ducts with sacs containing the appendages of the branchial hearts. The renal sacs communicate with the pericardium by pores near the external renal apertures; in the Octopoda the reno-pericardial openings are in the capsules of the branchial hearts. The genital ducts pass from the genital coelom to the exterior. They are paired in female Oigopsida and Octopoda exceptCirrhoteuthidae, but only the left persists in the males of all Dibranchiata, and in the female Myopsida.In the oviduct is a glandular enlargement, and in addition to this the females are provided with the so-called nidamental glands which are developed on the somatic wall of the pallial cavity, one on each side of the rectum, except in certain Oigopsida (Enoploteuthis, Cranchia, Leachia) and in the Octopoda, in which these organs are absent. The latter fact is related to the habit of the majority of the Octopoda of guarding or “incubating” their eggs, which have little protective covering. In the other cases the eggs are surrounded by a tough gelatinous elastic material secreted by the nidamental glands.The vas deferens is at first narrow and convoluted, then dilates into a vesicula seminalis at the end of which is a glandular diverticulum called the prostate. By the vesicula and the prostate the spermatophores are formed. These have a structure similar to those ofNautilus, and in the Octopoda may be as much as 50 mm. in length. Beyond the prostate the duct opens into a large terminal reservoir which has been called Needham’s sac, and in which the spermatophores are stored.Nervous System and Sense-Organs.—The figures (30, 31, 32) representing the nerve-centres ofOctopusserve to exhibit the dispositionof these parts in the Dibranchiata. The ganglia are more distinctly swollen than inNautilus. InOctopusan infra-buccal ganglion-pair are present, corresponding to the buccal ganglion-pair of Gastropoda. In Decapoda a supra-buccal ganglion-pair connected with these are also developed. Instead of the numerous radiating pallial nerves ofNautilus, we have in the Dibranchiata on each side (right and left) a large pleural nerve passing from the pleural portion of the pleuro-visceral ganglion to the mantle, where it enlarges to form the stellate ganglion. From each stellate ganglion nerves radiate to supply the powerful muscles of the mantle-skirt. The two stellate ganglia are connected, except inSepiola, by a transverse supra-oesophageal commissure, which represents the pallial cords united by a commissure above the intestine in Amphineura. The nerves from the visceral portion of the pleuro-visceral ganglion have the same course as inNautilus, but no osphradial papilla is present. An enteric nervous system is richly developed in the Dibranchiata, connected with the somatic nervous centres through the buccal ganglia, as in the Arthropoda through the stomato-gastric ganglia, and anastomozing with deep branches of the visceral nerves of the viscero-pleural ganglion-pair. It has been especially described by A. Hancock inOmmatostrephes. Upon the stomach it forms a single large and readily detected gastric ganglion.Fig.33.—Horizontal section of the eye ofSepia(Myopsid). (From Gegenbaur, after Hensen.)KK, Cephalic cartilages (see fig. 8).C, Cornea (closed).L, Lens.ci, Ciliary body.Ri, Internal layer of the retina.Re, External layer of the retina.p, Pigment between these.o, Optic nerve.go, Optic ganglion.kandk′, Capsular cartilage.ik, Cartilage of the iris.w, White body.ae, Argentine integument.Fig.34.—Diagrams of sections showing the early stage of development of the eye ofLoligowhen it is, like the permanent eye ofNautilusand ofPatella, an open sac. (From Lankester.)A, First appearance of the eye as a ring-like upgrowth.B, Ingrowth of the ring-like wall so as to form a sac, the primitive optic vesicle ofLoligo.In the Dibranchiate division of the Cephalopoda the greatest elaboration of the dioptric apparatus of the eye is attained, so that we have in this class the extremes of the two lines of development of the Molluscan eye, those two lines being the punctigerous and the lentigerous. The structure of the Dibranchiate’s eye is shown in section in fig. 14, C, and in fig. 33, and its development in figs. 34 and 37. The open sac which forms the retina of the young Dibranchiate closes up, and constitutes the posterior chamber of the eye, or primitive optic vesicle (fig. 37, A,poc). The lens forms as a structureless growth, secreted by both the internal and external surfaces of the front wall of the optic vesicle (fig. 37, B,l). The integument around the primitive optic vesicle which has sunk below the surface now rises up and forms firstly nearest the axis of the eye the iridian folds (ifin B, fig. 37;ikin fig. 33;Irin fig. 14), and then secondly an outer circular fold grows up like a wall and completely closes over the iridian folds and the axis of the primitive vesicle (fig. 33, C). This covering is transparent, and is the cornea. In the oceanic Decapoda the cornea does not completely close, but leaves a central aperture traversed by the optic axis. These forms are termed Oigopsidae by C. d’Orbigny, whilst the Decapoda with closed cornea are termed Myopsidae. In the Octopoda the cornea is closed, and there is yet another fold thrown over the eye. The skin surrounding the cornea presents a free circular margin, and can be drawn over the surface of the cornea by a sphincter muscle. It thus acts as an adjustable diaphragm, exactly similar in movement to the iris of Vertebrates.Sepiaand allied Decapods have a horizontal lower eyelid, that is to say, only one-half of the sphincter-like fold of integument is movable. The statocysts are situated ventrally between the pedal and visceral ganglia, and are entirely enclosed in the cranial cartilage. The cavity of each is continued into a small blind process which is the remnant of the embryonic connexion of the vesicle with the external surface. The sensory epithelium is at the anterior end of the vesicle forming a macula acustica, and in the cavity is a single otolith, partly calcareous and partly organic except inEledone, in which it is entirely organic. The nerve arises from the cerebral ganglion on each side and passes through the pedal ganglion.There is no branchial osphradium in the Dibranchiata corresponding to that ofNautilus, but the olfactory organ or rhinophore near the eye is present. InSepiaand the majority of the Dibranchiata it is a simple pit, in some of the Oigopsida it is a projection which may be stalked.Reproduction and Development.—The modification of one or a pair of the arms in the male for purposes of copulation has already been described. In many genera the sexes differ from one another in other characters also. As a rule the males are more slender or smaller than the females. The maximum degree of sexual dimorphism occurs inArgonautaamong the Octopods; in this genus the female may be fifteen times as large as the male, and the peculiar modification of the dorsal arms for the secretion of the shell occurs in the female only, no shell being formed in the male. In most cases the females are much more numerous than the males, but the opposite relation appears to exist in those Octopoda in which the hectocotylus is autotomous, for as many as four hectocotyli have been found in the pallial cavity of a single female. When the hectocotylus is not detached it is usually inserted into the pallial cavity of the female so as to deposit the spermatophores in or near the aperture of the oviduct, but inSepiaandLoligothey are merely deposited on the ventral lobes of the buccal membrane.The eggs are laid shortly after copulation. In the Octopoda and inSepia,SepiolaandRossia, each egg has a separate envelope continued into a long stalk by which it is attached with several others in a cluster. InArgonautathe eggs are carried by the female in the cavity of the shell. InLoligothe eggs are very numerous, and are enclosed in cylindrical transparent gelatinous strings united at one end into a cluster.The Cephalopoda appear to be the only Invertebrates in which the egg is mesoblastic and telolecithal like that of Vertebrata. This is the result of the large quantity of the yolk, and the position the latter assumes in relation to the blastoderm. In all other Mollusca the segmentation is complete though in some cases very unequal. In the egg ofLoligo, which has been chiefly studied (fig. 35), the protoplasmic pole is at the narrower end of the egg, and segmentation is restricted to this end, forming a layer of ectoderm cells. From one part of the periphery of the ectoderm proliferation of cells takes place and gives rise to a layer of scattered nuclei over the whole surface of the yolk. The region of proliferation marks the anal side of the ectoderm, and the layer of nuclei forms the perivitelline membrane. This process must be regarded as equivalent to the first stage of invagination, the yolk being surrounded by hypoblast cells or their nuclei. Later on the same anal edge of the ectoderm forms another cellular layer, the endoderm proper, which forms a continuous sheet below the ectoderm.The mesoderm also originates at the anal side of the ectoderm and extends in two bands right and left between ectoderm and endoderm. After the mesoderm is thus established, a little vesicle lying upon and open to the yolk is formed from the endoderm, and this vesicle ultimately gives rise to the stomach, the two lobes of the liver and the intestine. The buccal mass and oesophagus arise from a stomodaeal invagination, and the anus is formed later from a short proctodaeal invagination.Fig.35.—Development ofLoligo.1. View of the cleavage of the egg during the first formation of embryonic cells.2. Lateral view of the egg at a little later stage.a, Limit to which the layer of cleavage-cells has spread over the egg;b, portion of the egg (shaded) as yet uncovered by cleavage-cells;ap, the auto-plasts;kp, cleavage-pole where first cells were formed.3. Later stage, the limit (a) now extended so as to leave but little of the egg-surface (b) unenclosed. The eyes (d), mouth (e) and mantle-sac (u) have appeared.4. Later stage, anterior surface, the embryo is becoming nipped off from the yolk-sac (g).5. View of an embryo similar to (3) from the cleavage-pole or centro-dorsal area.6. Later stage, posterior surface.7. Section in a median dorso-ventral and antero-posterior plane of an embryo of the same age as (4).8. View of the anterior face of an older embryo.9. View of the posterior face of an embryo of the same age as (8).Letters in (3) to (9):—a, lateral fins of the mantle;b, mantle-skirt;c, supra-ocular invagination to form the “white body”;d, the eye;e, the mouth;f1,f2,f3,f4,f5, the five paired processes of the fore-foot; g, rhythmically contractile area of the yolk-sac, which is itself a hernia-like protrusion of the median portion of the fore-foot;h, dotted line showing internal area occupied by yolk (food-material of the egg);k, first rudiment of the epipodia (paired ridges which unite to form the siphon or funnel);l, sac of the radula or lingual ribbon;m, stomach;n, rudiments of the gills (paired ctenidia);o, the otocysts—a pair of invaginations of the surface of the epipodia;p, the optic ganglion;q, the distal portion of the ridges which form the siphon,kbeing the basal portion of the same structure;r, the vesicle-like rudiment of the intestine formed independently of the parts connected with the mouth,s,k,m, and without invagination;s, rudiment of the salivary glands;tin (7), the shell-sac at an earlier stage open (see fig. 36), now closed up;u, the open shell-sac formed by an uprising ring-like growth of the centro-dorsal area;win (5), the mantle-skirt commencing to be raised up around the area of the shell-sac. In (7)mespoints to the middle cell-layer of the embryo,epto the outer layer, andhto the deep layer of fusiform cells which separates everywhere the embryo from the yolk or food-material lying within it.The external changes of form are as follows:—The mantle is the middle of the embryonic area, and in its centre is the shell-gland, which, however, behaves in a different way from that seen in other Molluscs. Its borders grow inwards and approach each other to form the shell-sac. E. Ray Lankester showed that inArgonautaand other Octopods the shell-sac disappears before it is closed up, but in other forms exceptSpirulait closes completely and the shell develops within it. The lateral and posterior borders of the embryo form the foot, and these borders grow out into ten or eight lobes which become the arms, and which at first, as seen in fig. 35 (8), are entirely posterior to the mouth. Development actually shows the anterior arms gradually growing round the mouth and uniting in front of it.Between the mantle and the foot are two ridges which form the funnel, and their position shows them to be the epipodia. The otocysts and eyes are formed as invaginations of ectoderm, the former behind the eyes, at the sides of the funnel. All the nerve-centres, cerebral, visceral, pedal and optic, are formed as proliferations of the ectoderm. At the sides of the optic ganglia a pair of ectodermic invaginations are formed, which in the adult become the white bodies of the eyes, surrounding the optic ganglion. These are vestiges of lateral cerebral lobes which degenerate in the course of development.The coelomic cavity appears as a symmetrical pair of spaces in the mesoderm, right and left of the intestine, and from it grow out the genital ducts and the renal organs. The gonad develops from the wall of the coelom.Fig.36.—Section through aboral end of embryo ofLoligoshowing shell-sac still open.ep, ectoderm;m, mesoderm;m′, endoderm;shs, shell-sac;y, yolk.Phylogeny and Classification.—The order is divided into two sub-orders, Decapoda and Octopoda, by the presence or absence of the tentacular arms. The Decapoda are more adapted for swimming than the Octopoda, the body being usually provided with fins. In the former also there is generally an internal shell of considerable size, often calcined, while in the Octopoda only the merest vestiges of a shell remain. There can be no doubt that the Octopoda were derived from the Decapoda, although from the absence of skeletal structures fossil remains of Octopods are almost entirely unknown.Palaeoctopus, however, occurs in the Cretaceous, while shells ofArgonautado not appear before the Pliocene. The Decapoda are abundantly represented in the Secondary formations by theBelemnitidae, whose shell (fig. 19) consists of a straight conical phragmacone covered posteriorly by a very thick rostrum, and produced anteriorly into a thin long proöstracum which is only occasionally preserved. In certain cases remains of the arms provided with hooks, and of the ink-sac, have been recognized. TheBelemnitidaeappear first in the Upper Trias, attain their maximum development in the Jurassic rocks, and are not continued into the Tertiary period, though represented in the Eocene by a few allied forms.There is no difficulty in deriving the typical existing Decapoda fromBelemnitidae, and many of the extinct forms may have been directly ancestral. Chitinous “pens” like that ofLoligo, however, begin to appear in the Jurassic and Cretaceous rocks, so that in this case as in many others the parent form and the modified form existed contemporaneously, and the latter alone has survived. The oldest shells of theSepiatype are from the Eocene, and it is perhaps possible that theSepiidaearose separately from the Belemnites.It is a curious fact that no fossil specimens of the genusSpirulahave been found, but this may be due to the fact that it occurs only in deep water. At any rate there is no evidence that the shell ofSpirulahas lost a rostrum and a proöstracum; its characters must be regarded as primitive, not secondary. In the characters of the protoconch and of the commencement of the siphuncle, the shell ofSpirulaagrees with that of the Ammonoids, and in both its position is ventral, although in most Ammonoids the shell being exogastric the ventral side is the convex or external, while inSpirulathe shell is endogastric and the siphuncle internal. The fact that the shell is not completely enclosed by the mantle is also a primitive character.With regard to the general morphology of the Cephalopoda, it is difficult to reconcile the existence of two pairs of renal tubes as well as a pair of genital ducts inNautiluswith the view that the original Mollusc was unsegmented and had only one pair of coelomoducts. Considering the great specialization, however, and high degree of organization of the Cephalopods, it is evident that the earliest Nautiloid whose remains are known to us must have had a long evolutionary history behind it, and such metamerism as exists may have been developed in the course of its own history. In the other direction the evidence seems to prove that the Dibranchiata with only two renal ducts have been derived from the Tetrabranchiata.Suborder 1. Decapoda.—Four pairs of ordinary non-retractile arms which are shorter than the body, and one pair of tentacular arms, situated between the third and fourth normal arms on each side and retractile within special pouches. Suckers pedunculated and provided with horny rings, on the tentacular arms confined usually to the distal extremities. Usually a well-developed internal shell, and lateral fins on the edges of the body. Heart in a coelomic cavity; nidamentary glands usually present.Fig.37.—Right and left sections through embryos ofLoligo. (After Lankester.)A, Same stage as fig. 35 (4).B, Same stage as fig. 35 (8); only the left side of the sections is drawn, and the food-material which occupies the space internal to the membraneymis omitted.al, Rectum.is, Ink-sac.ep, Outer cell-layer.mes, Middle cell-layer.ym, Deep cell-layer of fusiform cells (yolk-membrane).ng, Optic nerve-ganglion.ot, Otocyst.wb, The “white body” of the adult ocular capsule forming as an invagination of the outer cell-layer.mtf, Mantle-skirt.g, Gill.ps, Pen-sac or shell-sac, now closed.dg, Dorsal groove.poc. Primitive optic vesicle, now closed (see fig. 34).l, Lens.r, Retina.soc, Second or anterior optic chamber still open.if, Iridean folds.C, The primitive invagination to form one of the otocysts, as seen in fig. 35 (5) and (6).Tribe 1.Oigopsida.—A wide aperture in the cornea. Two oviducts in the female. In fossil genera andSpirula, shell has a multilocular phragmacone with a siphuncle; initial chamber globular and larger than the second chamber. The most ancient forms characterized by the small size of the rostrum and proöstracum, and large size of the phragmacone. In the living genera, exceptSpirula, the shell is a chitinous gladius.Fam. 1.Belemnoteuthidae. Extinct; shell with well-developed phragmacone, and rostrum merely a calcareous envelope; siphuncular necks directed backwards as in Nautiloidea; ten equal arms provided with hooks.Phragmoteuthis, Trias.Belemnoteuthis, Jurassic and Cretaceous.Acanthoteuthis, Jurassic.Fam. 2.Aulacoceratidae. Extinct; phragmacone with widely separated septa; rostrum well developed and claviform.Aulacoceras, Trias.Atractites, Trias and Jurassic.Xiphoteuthis, Lias.Fam. 3.Belemnitidae. Extinct; phragmacone short with ventral siphuncle, prolonged dorsally into long proöstracum; rostrum large and cylindrical.Belemnites, 350 species from Jurassic and Cretaceous.Diploconus, Upper Jurassic.Fam. 4.Belopteridae. Extinct; rostrum and phragmacone well developed, phragmacone often curved; initial chamber small.Beloptera, Eocene.Bayanoteuthis, Eocene.Spirulirostra, Miocene.Fam. 5.Spirulidae. Dorsal and ventral sides of posterior extremity of shell uncovered by mantle; no rostrum or proöstracum; shell calcareous, coiled endogastrically and sipnunculated; fins posterior.Spirula, three living species known, abyssal.Fam. 6.Ommatostrephidae. Shell internal and chitinous, ending aborally in a little narrow cone; tentacular arms short and thick; suckers with denticulate rings.Ommatostrephes, fins aboral, simple and rhomboidal, British.Ctenopteryx, fins pectinate, as long as the body;Bathyteuthis, fins terminal, rudimentary; tentacular arms, filiform; abyssal.Rhynchoteuthis, tentacular arms united to form a beak-shaped appendage.Symplectoteuthis. Tracheloteuthis. Doridicus. Architeuthis; this is the largest of Cephalopoda, reaching 60 ft. in length including arms.Fam. 7.Thysanoteuthidae. Arms enlarged, bearing two rows of suckers and filaments; fins triangular, extending whole length of body.Thysanoteuthis, Mediterranean.Fam. 8.Onychoteuthidae. Fins terminal; tentacular arms long; suckers with hooks.Onychoteuthis, hook-bearing suckers on tentacular arms only.Enoploteuthis, hook-bearing suckers on all the arms.Veranya, body very short, tentacular arms atrophied in the adult, Mediterranean.Chaunoteuthis, body elongated, tentacular arms atrophied.Pterygioteuthis. Ancistroteuthis. Abralia. Teleoteuthis. Lepidoteuthis.Fam. 9.Gonatidae. Body elongated; fins terminal; radula with only two lateral teeth.Gonatus.Fam. 10.Cheiroteuthidae. Tentacular arms long, not retractile; resisting apparatus well developed.Cheiroteuthis, suckers along the whole length of the tentacular arms.Doratopsis, body very long and slender with aboral spine, dorsal arms very short.Histioteuthis, six dorsal arms united by membrane, photogenous organs present.Histiopsis, membrane of dorsal arms only half-way up the arms, photogenous organs present.Calliteuthis, no brachial membrane, photogenous organs present.Grimalditeuthis, two fins on each bide, no tentacular arms.Fam. 11.Cranchiidae. Eight normal arms, very short; eyes prominent; fins small and terminal.Cranchia, body short, purse-shaped, normal arms short, fins entirely aboral.Loligopsis, body elongated, conical, tentacular arms slender.Leachia, tentacular arms absent, funnel without a valve.Taonius, body elongated, normal arms, rather short, eyes pedunculated.Fig.38.—Octopodous Cephalopods.A,Pinnoctopus cordiformis, Quoy and Gain (from New Zealand).B,Tremoctopus violaceus, Ver. (from the Mediterranean).C,Cranchia scabra, Owen (from the Atlantic Ocean; one of the Decapoda).D,Cirrhoteuthis Mulleri, Esch. (from the Greenland coast).Tribe 2.Myopsida.—No aperture in the cornea. Left oviduct only developed in female. Internal shell without a distinct phragmacone, calcified or simply chitinous.Fam. 1.Sepiidae. Body wide and flat; fins narrow, extending the whole length of the body; shell calcareous and laminated.Belosepia, a rudiment of rostrum and phragmacone present in shell, Eocene.Sepia, shell with a rostrum, British.Sepiella, shell without a rostrum.Fam. 2.Sepiolidae. Body short, rounded at the aboral end; fins rounded, inserted in middle of body-length; shell chitinous, small or absent.Sepiola, head united to mantle dorsally, British.Rossia, head not united to mantle, British.StoloteuthisandInioteuthis, without shell.Heteroteuthis. Euprymna.Fam. 3.Idiosepiidae. Body elongated, with rudimentary terminal fins; internal shell almost lost.Idiosepius, 1.5 cm. long, Indian Ocean.Fam. 4.Sepiadariidae. Body short; mantle united to head dorsally; no shell.Sepiadarium, Pacific Ocean.Sepioloidea, Australian.Fam. 5.Loliginidae. Body elongated and conical; fins extending forward beyond the middle of body-length; shell chitinous, well developed.Loligo, fins triangular, aboral, British.Sepioteuthis, fins rounded, extending along whole of body-length.Loliolus. Loliguncula.The following fossil genera, known only by their gladius and ink-sac, have been placed nearLoligo:—Teuthopsis, BeloteuthisandGeoteuthis, Lias;Phylloteuthis, Cretaceous;Plesioteuthis, Jurassic and Cretaceous.Suborder 2. Octopoda.—Only four pairs of arms, all similar and longer than the body. Body short and rounded aborally. Suckers sessile. Heart not contained in coelom. No nidamentary glands.Fig.39.—Palaeoctopus Newboldi, the oldest Octopod known. From the Cretaceous rocks of Lebanon. (After H. Woodward.)Tribe I.Leioglossa.—No radula. Arms united by a complete membrane. Fins on sides of body.Fam.Cirrhoteuthidae. Tentacular filaments on either side of the suckers.Cirrhoteuthis, pallial sac prominent, fins large, pelagic.Opisthoteuthis, body flattened, with small fins, deep-sea.Vampyroteuthis, four fins.Palaeoctopus, fossil, Cretaceous.Tribe 2.Trachyglossa.—Radula present. No fins.Fam. 1.Amphitretidae. Arms united by membrane; funnel attached to mantle, dividing the pallial aperture into two.Amphitretus, pelagic.Fam. 2.Alloposidae. All arms united by membrane; mantle joined to head by dorsal band and two lateral commissures.Alloposus, pelagic.Fam. 3.Octopodidae. Arms long and equal, without membrane; hectocotylus not autotomous. No cephalic aquiferous pores.Octopus, two rows of suckers on each arm, British.Eledone, single row of suckers on each arm.Scaeurgus. Pinnoctopus. Cistopus. Japetella.Fam. 4.Philonexidae. Hectocotylus autotomous; arms unequal in size; aquiferous pores on head and funnel.Tremoctopus, two dorsal pairs of arms united by membrane.Ocythoë, without interbrachial membrane.Fam. 5.Argonautidae. Hectocotylus autotomous; no interbrachial membrane; extremities of dorsal arms in female expanded and secreting a shell; males very small, without shell.Argonauta.Literature.—Use has been freely made above of the article by E. Ray Lankester, onMollusca, in the 9th edition of this Encyclopedia. For the chief modern works, see Bashford Dean, “Notes on Living Nautilus,”Amer. Nat.xxxv., 1901; Arthur Willey, “Contribution to the Natural History of the Pearly Nautilus,” A. Willey’sZoological Results, pt. vi. (1902); Foord,Cat. Fossil Cephalopoda in British Museum; Alpheus Hyatt, “Fossil Cephalopods of the Museum of Comp. Zoology,”Bull. Mus. Comp. Zool.(Cambridge, U.S., 1868); Jalta, “I Cefalopodi viventi nel golfo di Napoli,”Fauna und Flora des Golfes von Neapel, xxiii. (1896); Joubin, “Céphalopodes de l’atlantique nord,” “Céph. de la Princesse Alice,”Camp. sci. Albert Ierde Monaco, ix. (1895), xxii. (1900); Paul Pelseneer, “Mollusca,” in theTreatise on Zoology, edited by E. Ray Lankester.
Head, Foot, Mantle and Mantle-cavity.—If we now compare the fore-foot of the Dibranchiata with that ofNautilus, we find in the first place a more simple arrangement of its lobes, which are either four or five pairs of tapering processes (called “arms”), arranged in a series around the buccal cone, and a substitution of suckers for tentacles on the surface of these lobes (figs. 15 and 24). The most dorsally placed pair of arms, corresponding to the two sides of the hood ofNautilus, are in reality the most anterior, and are termed the first pair. In the Octopoda there are four pairs of these arms (fig. 38), in the Decapoda five pairs, of which the fourth is greatly elongated (figs. 15, 16). InSepia,SepiolaandRossia, each of these long arms is withdrawn into a pouch beside the head, and is only ejected for the purpose of prehension. InLoligothey are completely retractile, very slightly so in the majority of the Oigopsida, and inRhynchoteuthisthey are united to form a beak-like appendage. A gradual reduction of the tentacular arms can be seen in the Decapoda, leading to their total absence in Octopoda; thus inLeachia,Chaunoteuthisand others these arms are reduced to mere stumps. In someCheiroteuthidaeandCranchiidaethe ordinary or sessile arms, especially the dorsal pairs, are reduced. In the Octopoda they are not unfrequently connected by a web, and form an efficient swimming-bell,e.g.inCirrhoteuthidaeandAmphuretidae. The suckers are placed on the adoral surface of the arms, and may be in one, two or four rows, and very numerous. In place of suckers in some genera,e.g.Veranya, we find on certain arms or parts of the arms horny hooks; in other cases a hook rises from the centre of each sucker. The hooks on the long arms ofOnychoteuthisare drawn in fig. 23. In various species ofCheiroteuthisthe suckers on the tentacular arms are very feeble, but the bottom of the cup is covered by a number of anastomosed epithelial filaments which are used as a fishing-net. The fore-foot, with its apparatus of suckers and hooks, is in the Dibranchiata essentially a prehensile apparatus, though the whole series of arms in the Octopoda serve as swimming organs, and in many (e.g.the common octopus or poulp) the sucker-bearing surface is used as a crawling organ.
C, The head.
J, The mid-foot or siphon, which has been cut open so as to display the valvei.
R, The glandular tissue of the left nephridium or renal-sac, which has been cut open (see fig. 29).
P, P, The lateral fins of the mantle-skirt.
Br, The single pair of branchiae (ctenidia).
a, The anus—immediately below it is the opening of the ink-bag.
c, Cartilaginous socket in the siphon to receivec′, the cartilaginous knob of the mantle-skirt—the two constituting the “pallial hinge apparatus” characteristic of Decapoda, not found in Octopoda.
g, The azygos genital papilla and aperture.
′i, Valve of the siphon (possibly the rudimentary hind-foot)
m, Muscular band connected with the fore-foot and mid-foot (siphon) and identical with the muscular masskin fig. 3.
r, Renal papillae, carrying the apertures of the nephridia.
v.br, Branchial efferent blood-vessel.
v br′, Bulbous enlargements of the branchial blood-vessels (see figs 28, 29).
t, Ink-bag.
In the males of the Dibranchiata one of the arms is more or less modified in connexion with the reproductive function, and is called the “hectocotylized arm.” This name is derived from the condition assumed by the arm in those cases in which its modification is carried out to the greatest extent. These cases are those of the OctopodsArgonauta argoandOcythoe catenulata(fig. 24). In the males of these the third arm (on the left side inArgonauta, on the right side inOcythoe) is found before the breeding season to be represented by a globular sac of integument. This sac bursts, and from it issues an arm larger than its neighbours, having a small sac at its extremity inOcythoe(fig. 24.x), from which subsequently a long filament issues. Before copulation the male charges this arm with the spermatophores or packets of spermatozoa removed from its generative orifice beneath the mantle-skirt, and during coitus the arm becomes detached and is left adhering to the female by means of its suckers. A new arm is formed at the cicatrix before the next breeding season. The female, being much larger than the male, swims away with the detached arm lodged beneath her mantle-skirt. There, in a way which is not understood, the fertilization of the eggsis effected. Specimens of the femaleOcythoëwith the detached arm adherent were examined by Cuvier, who mistook the arm for a parasitic worm and gave to it the nameHectocotylus. Accordingly, the correspondingly modified arms of other Cephalopoda are said to be hectocotylized. J.J.S. Steenstrup has determined the hectocotylized condition of one or other of the arms in a number of male Dibranchs as follows:—in all, exceptingArgonautaandOcythoeandTremoctopus, the modification of the arm is slight, consisting in a small enlargement of part or the whole of the arm, and the obliteration of some of its suckers; inOctopusandEledonethe third right arm is hectocotylized; inRossiaandSepiolathe fourth left arm is hectocotylized along its whole length, and the fourth right arm also in the middle only; inSepiathe fourth left arm is modified at its base only; inSepioteuthis, the same at its apex; inLoligo, the same also at its apex; inLoliolus, the same along its whole length; inOmmatostrephes,OnychoteuthisandLoligopsisno hectocotylized arm has hitherto been observed. Thus, speaking generally, it is one or both of the fourth pair of short arms which are modified in the Decapoda, of the third pair in the Octopoda. In the pallial cavity are situated one pair of gills in the Dibranchiata (fig. 25), attached dorsally along the whole of their afferent borders. On each side of the branchia is a series of lamellae, least in number in the Octopoda. Each lamella is transversely folded, and the folds are in turn folded, so that the respiratory surface is increased. On the somatic wall of the pallial cavity, between and ventral to the gills, are the following apertures: the anus and opening of the ink-sac, close together in the median line; a pair of apertures of the renal sacs, on either side of the median line; external to the renal orifice, on the left side, the genital aperture inCirrhoteuthidaeand Myopsida. In other Octopoda, and in nearly all the Oigopsida among the Decapoda, the genital ducts are paired in the female, but only the left is developed in the male. The funnel forms a complete tube in the Dibranchiata, and in the majority of the Decapoda, as inNautilus, it is provided with an internal valve projecting from its somatic surface, which allows water to pass outwards but prevents it passing inwards. The mantle performs rhythmical respiratory movements of expansion and contraction, the water entering between funnel and mantle and passing out through the funnel. In Decapoda the edge of the mantle bears internally on each side a cartilaginous projection which fits into a corresponding depression on the external surface of the funnel; this is called the “resisting apparatus,” and serves to make the union of mantle and funnel firmer during expiration. More powerful expiratory movements are used for sudden retrograde locomotion through the water.
a, Shell (here enclosed by a growth of the mantle).
b, The nuchal plate (here a cartilage).
c, (The reference line should be continued through the black area representing the shell to the outline below it), the integument covering the visceral hump.
d, The reflected portion of the mantle-skirt forming the sac which encloses the shell.
e, The inferior margin of the mantle-skirt (mouth of the pallial chamber).
f, The pallial chamber.
g, The vertically cut median portion of the siphon.
i, The valve of the siphon.
m, The two upper lobes of the fore-foot.
n, The long prehensile arms of the same.
o, The fifth or lowermost lobe of the fore-foot.
p, The third lobe of the fore-foot.
q, The buccal membrane.
v, The upper beak or jaw.
s, The lower beak or jaw.
t, The lingual ribbon.
x, The viscero-pericardial sac.
n.c, The nerve-collar.
cr, The crop.
gizz, The gizzard.
an, The anus.
c.t, The left ctenidium or gill-plume.
vent, Ventricle of the heart.
a.b.v, Afferent branchial vessel.
e.b.v, Efferent branchial vessel.
re, Renal glandular mass.
n.n.a, Left nephridial aperture.
visc.per.apert, Viscero-pericardial aperture (see fig. 29).
br.b, Branchial heart.
app, Appendage of the same.
i.s, Ink-bag.
Luminous Organs.—In certain Oigopsida living in deep water,e.g.Histioteuthis, Calliteuthis, Histiopsis, Pterygioteuthis, the surface of the skin bears photogenous organs directed towards the oral extremity. Anatomically these consist of a deeper photogenous layer and a more superficial refracting layer. In some cases,e.g.Pterygioteuthis, they occur even within the mantle-cavity.
Fins.—In the majority of the Decapoda and in theCirrhoteuthidae, the mantle is produced into lateral symmetrical expansions which have the function of fins. They originate at the aboral extremity where they remain inSpirula(fig. 18). In most other Oigopsida they are terminal, but more dorsal than ventral,e.g.Loligopsis(fig. 16), and there may be two on each side, as inGrimalditeuthis. In other cases they extend laterally along a greater length of the body, as inSepia(fig. 15). InCtenopteryxthey have a superficial resemblance to the fins of fishes, consisting of a thin membrane supported by a series of muscular rods.
oe, Oesophagus.
v, The stomach opened longitudinally.
x, Probe passed through the pylorus.
c, Commencement of the caecum.
e, Its spiral portion.
i, Intestine.
a, Ink-bag.
b, Its opening into the rectum.
Chromatophores.—These are characteristic of the Dibranchiata, apparently absent inNautilus. They are originally single cells of ectodermic origin which sink below the epidermis and become connected with radiating muscular fibres. The cells are single but multinuclear. Different cells contain pigments of different colours, yellow, brown, red or blue. Each cell in life is in constant tremulous movement; under the influence of nervous excitement the cells are suddenly expanded or contracted, producing blushes of colour and pallor. By reflex action of which the afferent stimulus acts upon the eyes as in fishes, the chromatophores assume a condition which approximates the colour of the animal to that of surrounding objects. In the Decapoda there are also reflecting elements which produce iridescent hues.
Aquiferous Cavities.—In addition to the pockets into which the tentacular arms of Decapoda are retracted, there are in several Dibranchiata cavities in the integument which open to the exterior by special pores but have no communication with the vascular system or other internal cavities of the body. InOcythoethere are such pores on the back of the head and at the base of the funnel; buccal pouches on the ventral side of the mouth, internal to the arms, occur in some genera, one inLoligo, two inSepia. In some species ofSepiathere are pouches in the mantle.
Alimentary Tube.—The principal differences fromNautilusare the following:—the mandibles are similar in shape, but are chitinous, not calcified. In the radula there are three teeth on each side of the median tooth in each row, except inGonatus, in which there are only two lateral teeth, and theCirrhoteuthidae, in which the radula has entirely disappeared. In front of the radula is the so-called tongue, a fleshy projection corresponding to the sub-radular organ of other Mollusca.
In most of the Dibranchiata there are two pairs of salivary glands. In the Decapoda the ducts of the posterior pair unite into a median duct which opens on the surface of the sub-radular organ. The anterior pair is but slightly developed except in the Oigopsida. In the Octopoda there are also two pairs, but the posterior pair, except inCirrhoteuthiswhere they are absent, are large and displaced backwards, being situated near the oesophageal proventriculus. Connected with the intestine immediately beyond the pylorus is a thin-walled caecum, spherical inRossiaandLeachia, elongated inLoligo, but usually coiled into a spiral (fig. 27). The hepatic ducts open into the caecum. The liver is developed as a paired gland, more or less fused into one in the adult, but the ducts are always paired. The ducts are covered by a number of glandular follicles forming what is called the pancreas.
The ink-sac, absent inNautilus, is a rectal caecum developed from its dorsal wall. It is present in all Dibranchiata exceptOctopus arcticus, O. piscatorumandCirrhoteuthis. It consists of a deeper part or gland proper and a reservoir. It extends to the posterior extremity of the body inSepia, but inOctopodais usually embedded in the surface of the liver. The pigment of the secretion is melanin, and its function is to produce a dense opacity in the water, which conceals the animal.
Vascular System(fig. 28).—The ventricle lies in the pericardial cavity, except in Octopoda where this cavity is much reduced. The auricles, one pair, are contractile expansions of the efferent branchial vessels. The heart gives off an anterior or cephalic and a posterior or abdominal aorta. The vascular system is almost perfect, arteries and veins being united by capillaries. The principal vein is a venacava passing backwards ventrally from the cephalic region and dividing into two afferent branchial veins, each of which receives a pallial and an abdominal vein. Each of these afferent branchial vessels is enclosed in the cavity of a renal organ and is covered externally by the glandular tissue which forms the excretory part of the “kidney” (fig. 29). Each afferent vessel is expanded into a contractile branchial heart, which is provided with a glandular appendage. The latter corresponds to the glandular masses which are attached to the afferent branchial veins inNautilus, and to the pericardial glands of other Molluscs.
br, Branchiae (ctenidia).
c, Ventricle of the heart.
a, Anterior artery (aorta).
a′, Posterior artery.
v, The right and left auricles (enlargements of the efferent branchial veins).
v′, Efferent branchial vein on the free face of the gill-plume.
v.c, Vena cava.
vi, vc′, Afferent branchial vessels (branches of the vena cava, see fig. 29).
vc″, Abdominal veins.
x, Branchial hearts and appendages.
re, e, Glandular substance of the nephridia developed on the wall of the great veins on their way to the gills. The arrows indicate the direction of the blood-current.
v.c, Vena cava.
r.d.v.c, Right descending branch of the same.
r.s.v.c, Left descending branch of the same.
v.b.a, Vein from the ink-bag.
v.m, Mesenteric vein.
v.g, Genital vein.
v.a.d, Right abdominal vein.
v.a.s, Left abdominal vein.
v.p.d, Right pallial vein.
v.p.s, Left pallial vein.
c.b, Branchial heart.
x, Appendage of the same.
c.v, Capsule of the branchial heart.
np, External aperture of the right nephridial sac.
y, Reno-pericardial orifice placing the left renal sac or nephridium in communication with the viscero-pericardial sac, the course of which below the nephridial sac is indicated by dotted lines.
y′, The similar orifice of the right side.
a.r, Glandular renal outgrowths.
w.k, Viscero-pericardial sac (dotted outline).
buc, The buccal mass.
ped, Pedal ganglion.
opt, Optic ganglion.
cer, Cerebral ganglion.
pl, Pleural ganglion.
visc, Visceral ganglion.
oes, Oesophagus.
f, Foramen in the nerve-mass formed by pedal, pleural and visceral ganglion-pairs, traversed by a blood-vessel.
bg, Buccal ganglion.
cer, Cerebral ganglion.
ped, Pedal ganglion.
pl, Pleural, and visc., visceral region of the pleuro-visceral ganglion.
gang. stell, The right stellate ganglion of the mantle connected by a nerve to the pleural portion.
n.visc, The right visceral nerve.
n.olf, Its (probably) olfactory branches.
n.br, Its branchial branches.
Coelom.—The coelom forms a large sac with a constriction between the anterior or pericardial division and the posterior or genital division, and it is produced into lateral diverticula which contain the branchial hearts; but in the Octopoda the pericardial division is suppressed and the genital division communicates by long ducts with sacs containing the appendages of the branchial hearts. The renal sacs communicate with the pericardium by pores near the external renal apertures; in the Octopoda the reno-pericardial openings are in the capsules of the branchial hearts. The genital ducts pass from the genital coelom to the exterior. They are paired in female Oigopsida and Octopoda exceptCirrhoteuthidae, but only the left persists in the males of all Dibranchiata, and in the female Myopsida.
In the oviduct is a glandular enlargement, and in addition to this the females are provided with the so-called nidamental glands which are developed on the somatic wall of the pallial cavity, one on each side of the rectum, except in certain Oigopsida (Enoploteuthis, Cranchia, Leachia) and in the Octopoda, in which these organs are absent. The latter fact is related to the habit of the majority of the Octopoda of guarding or “incubating” their eggs, which have little protective covering. In the other cases the eggs are surrounded by a tough gelatinous elastic material secreted by the nidamental glands.
The vas deferens is at first narrow and convoluted, then dilates into a vesicula seminalis at the end of which is a glandular diverticulum called the prostate. By the vesicula and the prostate the spermatophores are formed. These have a structure similar to those ofNautilus, and in the Octopoda may be as much as 50 mm. in length. Beyond the prostate the duct opens into a large terminal reservoir which has been called Needham’s sac, and in which the spermatophores are stored.
Nervous System and Sense-Organs.—The figures (30, 31, 32) representing the nerve-centres ofOctopusserve to exhibit the dispositionof these parts in the Dibranchiata. The ganglia are more distinctly swollen than inNautilus. InOctopusan infra-buccal ganglion-pair are present, corresponding to the buccal ganglion-pair of Gastropoda. In Decapoda a supra-buccal ganglion-pair connected with these are also developed. Instead of the numerous radiating pallial nerves ofNautilus, we have in the Dibranchiata on each side (right and left) a large pleural nerve passing from the pleural portion of the pleuro-visceral ganglion to the mantle, where it enlarges to form the stellate ganglion. From each stellate ganglion nerves radiate to supply the powerful muscles of the mantle-skirt. The two stellate ganglia are connected, except inSepiola, by a transverse supra-oesophageal commissure, which represents the pallial cords united by a commissure above the intestine in Amphineura. The nerves from the visceral portion of the pleuro-visceral ganglion have the same course as inNautilus, but no osphradial papilla is present. An enteric nervous system is richly developed in the Dibranchiata, connected with the somatic nervous centres through the buccal ganglia, as in the Arthropoda through the stomato-gastric ganglia, and anastomozing with deep branches of the visceral nerves of the viscero-pleural ganglion-pair. It has been especially described by A. Hancock inOmmatostrephes. Upon the stomach it forms a single large and readily detected gastric ganglion.
KK, Cephalic cartilages (see fig. 8).
C, Cornea (closed).
L, Lens.
ci, Ciliary body.
Ri, Internal layer of the retina.
Re, External layer of the retina.
p, Pigment between these.
o, Optic nerve.
go, Optic ganglion.
kandk′, Capsular cartilage.
ik, Cartilage of the iris.
w, White body.
ae, Argentine integument.
In the Dibranchiate division of the Cephalopoda the greatest elaboration of the dioptric apparatus of the eye is attained, so that we have in this class the extremes of the two lines of development of the Molluscan eye, those two lines being the punctigerous and the lentigerous. The structure of the Dibranchiate’s eye is shown in section in fig. 14, C, and in fig. 33, and its development in figs. 34 and 37. The open sac which forms the retina of the young Dibranchiate closes up, and constitutes the posterior chamber of the eye, or primitive optic vesicle (fig. 37, A,poc). The lens forms as a structureless growth, secreted by both the internal and external surfaces of the front wall of the optic vesicle (fig. 37, B,l). The integument around the primitive optic vesicle which has sunk below the surface now rises up and forms firstly nearest the axis of the eye the iridian folds (ifin B, fig. 37;ikin fig. 33;Irin fig. 14), and then secondly an outer circular fold grows up like a wall and completely closes over the iridian folds and the axis of the primitive vesicle (fig. 33, C). This covering is transparent, and is the cornea. In the oceanic Decapoda the cornea does not completely close, but leaves a central aperture traversed by the optic axis. These forms are termed Oigopsidae by C. d’Orbigny, whilst the Decapoda with closed cornea are termed Myopsidae. In the Octopoda the cornea is closed, and there is yet another fold thrown over the eye. The skin surrounding the cornea presents a free circular margin, and can be drawn over the surface of the cornea by a sphincter muscle. It thus acts as an adjustable diaphragm, exactly similar in movement to the iris of Vertebrates.Sepiaand allied Decapods have a horizontal lower eyelid, that is to say, only one-half of the sphincter-like fold of integument is movable. The statocysts are situated ventrally between the pedal and visceral ganglia, and are entirely enclosed in the cranial cartilage. The cavity of each is continued into a small blind process which is the remnant of the embryonic connexion of the vesicle with the external surface. The sensory epithelium is at the anterior end of the vesicle forming a macula acustica, and in the cavity is a single otolith, partly calcareous and partly organic except inEledone, in which it is entirely organic. The nerve arises from the cerebral ganglion on each side and passes through the pedal ganglion.
There is no branchial osphradium in the Dibranchiata corresponding to that ofNautilus, but the olfactory organ or rhinophore near the eye is present. InSepiaand the majority of the Dibranchiata it is a simple pit, in some of the Oigopsida it is a projection which may be stalked.
Reproduction and Development.—The modification of one or a pair of the arms in the male for purposes of copulation has already been described. In many genera the sexes differ from one another in other characters also. As a rule the males are more slender or smaller than the females. The maximum degree of sexual dimorphism occurs inArgonautaamong the Octopods; in this genus the female may be fifteen times as large as the male, and the peculiar modification of the dorsal arms for the secretion of the shell occurs in the female only, no shell being formed in the male. In most cases the females are much more numerous than the males, but the opposite relation appears to exist in those Octopoda in which the hectocotylus is autotomous, for as many as four hectocotyli have been found in the pallial cavity of a single female. When the hectocotylus is not detached it is usually inserted into the pallial cavity of the female so as to deposit the spermatophores in or near the aperture of the oviduct, but inSepiaandLoligothey are merely deposited on the ventral lobes of the buccal membrane.
The eggs are laid shortly after copulation. In the Octopoda and inSepia,SepiolaandRossia, each egg has a separate envelope continued into a long stalk by which it is attached with several others in a cluster. InArgonautathe eggs are carried by the female in the cavity of the shell. InLoligothe eggs are very numerous, and are enclosed in cylindrical transparent gelatinous strings united at one end into a cluster.
The Cephalopoda appear to be the only Invertebrates in which the egg is mesoblastic and telolecithal like that of Vertebrata. This is the result of the large quantity of the yolk, and the position the latter assumes in relation to the blastoderm. In all other Mollusca the segmentation is complete though in some cases very unequal. In the egg ofLoligo, which has been chiefly studied (fig. 35), the protoplasmic pole is at the narrower end of the egg, and segmentation is restricted to this end, forming a layer of ectoderm cells. From one part of the periphery of the ectoderm proliferation of cells takes place and gives rise to a layer of scattered nuclei over the whole surface of the yolk. The region of proliferation marks the anal side of the ectoderm, and the layer of nuclei forms the perivitelline membrane. This process must be regarded as equivalent to the first stage of invagination, the yolk being surrounded by hypoblast cells or their nuclei. Later on the same anal edge of the ectoderm forms another cellular layer, the endoderm proper, which forms a continuous sheet below the ectoderm.
The mesoderm also originates at the anal side of the ectoderm and extends in two bands right and left between ectoderm and endoderm. After the mesoderm is thus established, a little vesicle lying upon and open to the yolk is formed from the endoderm, and this vesicle ultimately gives rise to the stomach, the two lobes of the liver and the intestine. The buccal mass and oesophagus arise from a stomodaeal invagination, and the anus is formed later from a short proctodaeal invagination.
1. View of the cleavage of the egg during the first formation of embryonic cells.
2. Lateral view of the egg at a little later stage.a, Limit to which the layer of cleavage-cells has spread over the egg;b, portion of the egg (shaded) as yet uncovered by cleavage-cells;ap, the auto-plasts;kp, cleavage-pole where first cells were formed.
3. Later stage, the limit (a) now extended so as to leave but little of the egg-surface (b) unenclosed. The eyes (d), mouth (e) and mantle-sac (u) have appeared.
4. Later stage, anterior surface, the embryo is becoming nipped off from the yolk-sac (g).
5. View of an embryo similar to (3) from the cleavage-pole or centro-dorsal area.
6. Later stage, posterior surface.
7. Section in a median dorso-ventral and antero-posterior plane of an embryo of the same age as (4).
8. View of the anterior face of an older embryo.
9. View of the posterior face of an embryo of the same age as (8).
Letters in (3) to (9):—a, lateral fins of the mantle;b, mantle-skirt;c, supra-ocular invagination to form the “white body”;d, the eye;e, the mouth;f1,f2,f3,f4,f5, the five paired processes of the fore-foot; g, rhythmically contractile area of the yolk-sac, which is itself a hernia-like protrusion of the median portion of the fore-foot;h, dotted line showing internal area occupied by yolk (food-material of the egg);k, first rudiment of the epipodia (paired ridges which unite to form the siphon or funnel);l, sac of the radula or lingual ribbon;m, stomach;n, rudiments of the gills (paired ctenidia);o, the otocysts—a pair of invaginations of the surface of the epipodia;p, the optic ganglion;q, the distal portion of the ridges which form the siphon,kbeing the basal portion of the same structure;r, the vesicle-like rudiment of the intestine formed independently of the parts connected with the mouth,s,k,m, and without invagination;s, rudiment of the salivary glands;tin (7), the shell-sac at an earlier stage open (see fig. 36), now closed up;u, the open shell-sac formed by an uprising ring-like growth of the centro-dorsal area;win (5), the mantle-skirt commencing to be raised up around the area of the shell-sac. In (7)mespoints to the middle cell-layer of the embryo,epto the outer layer, andhto the deep layer of fusiform cells which separates everywhere the embryo from the yolk or food-material lying within it.
The external changes of form are as follows:—The mantle is the middle of the embryonic area, and in its centre is the shell-gland, which, however, behaves in a different way from that seen in other Molluscs. Its borders grow inwards and approach each other to form the shell-sac. E. Ray Lankester showed that inArgonautaand other Octopods the shell-sac disappears before it is closed up, but in other forms exceptSpirulait closes completely and the shell develops within it. The lateral and posterior borders of the embryo form the foot, and these borders grow out into ten or eight lobes which become the arms, and which at first, as seen in fig. 35 (8), are entirely posterior to the mouth. Development actually shows the anterior arms gradually growing round the mouth and uniting in front of it.Between the mantle and the foot are two ridges which form the funnel, and their position shows them to be the epipodia. The otocysts and eyes are formed as invaginations of ectoderm, the former behind the eyes, at the sides of the funnel. All the nerve-centres, cerebral, visceral, pedal and optic, are formed as proliferations of the ectoderm. At the sides of the optic ganglia a pair of ectodermic invaginations are formed, which in the adult become the white bodies of the eyes, surrounding the optic ganglion. These are vestiges of lateral cerebral lobes which degenerate in the course of development.
The coelomic cavity appears as a symmetrical pair of spaces in the mesoderm, right and left of the intestine, and from it grow out the genital ducts and the renal organs. The gonad develops from the wall of the coelom.
Phylogeny and Classification.—The order is divided into two sub-orders, Decapoda and Octopoda, by the presence or absence of the tentacular arms. The Decapoda are more adapted for swimming than the Octopoda, the body being usually provided with fins. In the former also there is generally an internal shell of considerable size, often calcined, while in the Octopoda only the merest vestiges of a shell remain. There can be no doubt that the Octopoda were derived from the Decapoda, although from the absence of skeletal structures fossil remains of Octopods are almost entirely unknown.Palaeoctopus, however, occurs in the Cretaceous, while shells ofArgonautado not appear before the Pliocene. The Decapoda are abundantly represented in the Secondary formations by theBelemnitidae, whose shell (fig. 19) consists of a straight conical phragmacone covered posteriorly by a very thick rostrum, and produced anteriorly into a thin long proöstracum which is only occasionally preserved. In certain cases remains of the arms provided with hooks, and of the ink-sac, have been recognized. TheBelemnitidaeappear first in the Upper Trias, attain their maximum development in the Jurassic rocks, and are not continued into the Tertiary period, though represented in the Eocene by a few allied forms.
There is no difficulty in deriving the typical existing Decapoda fromBelemnitidae, and many of the extinct forms may have been directly ancestral. Chitinous “pens” like that ofLoligo, however, begin to appear in the Jurassic and Cretaceous rocks, so that in this case as in many others the parent form and the modified form existed contemporaneously, and the latter alone has survived. The oldest shells of theSepiatype are from the Eocene, and it is perhaps possible that theSepiidaearose separately from the Belemnites.
It is a curious fact that no fossil specimens of the genusSpirulahave been found, but this may be due to the fact that it occurs only in deep water. At any rate there is no evidence that the shell ofSpirulahas lost a rostrum and a proöstracum; its characters must be regarded as primitive, not secondary. In the characters of the protoconch and of the commencement of the siphuncle, the shell ofSpirulaagrees with that of the Ammonoids, and in both its position is ventral, although in most Ammonoids the shell being exogastric the ventral side is the convex or external, while inSpirulathe shell is endogastric and the siphuncle internal. The fact that the shell is not completely enclosed by the mantle is also a primitive character.
With regard to the general morphology of the Cephalopoda, it is difficult to reconcile the existence of two pairs of renal tubes as well as a pair of genital ducts inNautiluswith the view that the original Mollusc was unsegmented and had only one pair of coelomoducts. Considering the great specialization, however, and high degree of organization of the Cephalopods, it is evident that the earliest Nautiloid whose remains are known to us must have had a long evolutionary history behind it, and such metamerism as exists may have been developed in the course of its own history. In the other direction the evidence seems to prove that the Dibranchiata with only two renal ducts have been derived from the Tetrabranchiata.
Suborder 1. Decapoda.—Four pairs of ordinary non-retractile arms which are shorter than the body, and one pair of tentacular arms, situated between the third and fourth normal arms on each side and retractile within special pouches. Suckers pedunculated and provided with horny rings, on the tentacular arms confined usually to the distal extremities. Usually a well-developed internal shell, and lateral fins on the edges of the body. Heart in a coelomic cavity; nidamentary glands usually present.
A, Same stage as fig. 35 (4).
B, Same stage as fig. 35 (8); only the left side of the sections is drawn, and the food-material which occupies the space internal to the membraneymis omitted.
al, Rectum.
is, Ink-sac.
ep, Outer cell-layer.
mes, Middle cell-layer.
ym, Deep cell-layer of fusiform cells (yolk-membrane).
ng, Optic nerve-ganglion.
ot, Otocyst.
wb, The “white body” of the adult ocular capsule forming as an invagination of the outer cell-layer.
mtf, Mantle-skirt.
g, Gill.
ps, Pen-sac or shell-sac, now closed.
dg, Dorsal groove.
poc. Primitive optic vesicle, now closed (see fig. 34).
l, Lens.
r, Retina.
soc, Second or anterior optic chamber still open.
if, Iridean folds.
C, The primitive invagination to form one of the otocysts, as seen in fig. 35 (5) and (6).
Tribe 1.Oigopsida.—A wide aperture in the cornea. Two oviducts in the female. In fossil genera andSpirula, shell has a multilocular phragmacone with a siphuncle; initial chamber globular and larger than the second chamber. The most ancient forms characterized by the small size of the rostrum and proöstracum, and large size of the phragmacone. In the living genera, exceptSpirula, the shell is a chitinous gladius.
Fam. 1.Belemnoteuthidae. Extinct; shell with well-developed phragmacone, and rostrum merely a calcareous envelope; siphuncular necks directed backwards as in Nautiloidea; ten equal arms provided with hooks.Phragmoteuthis, Trias.Belemnoteuthis, Jurassic and Cretaceous.Acanthoteuthis, Jurassic.Fam. 2.Aulacoceratidae. Extinct; phragmacone with widely separated septa; rostrum well developed and claviform.Aulacoceras, Trias.Atractites, Trias and Jurassic.Xiphoteuthis, Lias.Fam. 3.Belemnitidae. Extinct; phragmacone short with ventral siphuncle, prolonged dorsally into long proöstracum; rostrum large and cylindrical.Belemnites, 350 species from Jurassic and Cretaceous.Diploconus, Upper Jurassic.Fam. 4.Belopteridae. Extinct; rostrum and phragmacone well developed, phragmacone often curved; initial chamber small.Beloptera, Eocene.Bayanoteuthis, Eocene.Spirulirostra, Miocene.Fam. 5.Spirulidae. Dorsal and ventral sides of posterior extremity of shell uncovered by mantle; no rostrum or proöstracum; shell calcareous, coiled endogastrically and sipnunculated; fins posterior.Spirula, three living species known, abyssal.Fam. 6.Ommatostrephidae. Shell internal and chitinous, ending aborally in a little narrow cone; tentacular arms short and thick; suckers with denticulate rings.Ommatostrephes, fins aboral, simple and rhomboidal, British.Ctenopteryx, fins pectinate, as long as the body;Bathyteuthis, fins terminal, rudimentary; tentacular arms, filiform; abyssal.Rhynchoteuthis, tentacular arms united to form a beak-shaped appendage.Symplectoteuthis. Tracheloteuthis. Doridicus. Architeuthis; this is the largest of Cephalopoda, reaching 60 ft. in length including arms.Fam. 7.Thysanoteuthidae. Arms enlarged, bearing two rows of suckers and filaments; fins triangular, extending whole length of body.Thysanoteuthis, Mediterranean.Fam. 8.Onychoteuthidae. Fins terminal; tentacular arms long; suckers with hooks.Onychoteuthis, hook-bearing suckers on tentacular arms only.Enoploteuthis, hook-bearing suckers on all the arms.Veranya, body very short, tentacular arms atrophied in the adult, Mediterranean.Chaunoteuthis, body elongated, tentacular arms atrophied.Pterygioteuthis. Ancistroteuthis. Abralia. Teleoteuthis. Lepidoteuthis.Fam. 9.Gonatidae. Body elongated; fins terminal; radula with only two lateral teeth.Gonatus.Fam. 10.Cheiroteuthidae. Tentacular arms long, not retractile; resisting apparatus well developed.Cheiroteuthis, suckers along the whole length of the tentacular arms.Doratopsis, body very long and slender with aboral spine, dorsal arms very short.Histioteuthis, six dorsal arms united by membrane, photogenous organs present.Histiopsis, membrane of dorsal arms only half-way up the arms, photogenous organs present.Calliteuthis, no brachial membrane, photogenous organs present.Grimalditeuthis, two fins on each bide, no tentacular arms.Fam. 11.Cranchiidae. Eight normal arms, very short; eyes prominent; fins small and terminal.Cranchia, body short, purse-shaped, normal arms short, fins entirely aboral.Loligopsis, body elongated, conical, tentacular arms slender.Leachia, tentacular arms absent, funnel without a valve.Taonius, body elongated, normal arms, rather short, eyes pedunculated.
Fam. 1.Belemnoteuthidae. Extinct; shell with well-developed phragmacone, and rostrum merely a calcareous envelope; siphuncular necks directed backwards as in Nautiloidea; ten equal arms provided with hooks.Phragmoteuthis, Trias.Belemnoteuthis, Jurassic and Cretaceous.Acanthoteuthis, Jurassic.
Fam. 2.Aulacoceratidae. Extinct; phragmacone with widely separated septa; rostrum well developed and claviform.Aulacoceras, Trias.Atractites, Trias and Jurassic.Xiphoteuthis, Lias.
Fam. 3.Belemnitidae. Extinct; phragmacone short with ventral siphuncle, prolonged dorsally into long proöstracum; rostrum large and cylindrical.Belemnites, 350 species from Jurassic and Cretaceous.Diploconus, Upper Jurassic.
Fam. 4.Belopteridae. Extinct; rostrum and phragmacone well developed, phragmacone often curved; initial chamber small.Beloptera, Eocene.Bayanoteuthis, Eocene.Spirulirostra, Miocene.
Fam. 5.Spirulidae. Dorsal and ventral sides of posterior extremity of shell uncovered by mantle; no rostrum or proöstracum; shell calcareous, coiled endogastrically and sipnunculated; fins posterior.Spirula, three living species known, abyssal.
Fam. 6.Ommatostrephidae. Shell internal and chitinous, ending aborally in a little narrow cone; tentacular arms short and thick; suckers with denticulate rings.Ommatostrephes, fins aboral, simple and rhomboidal, British.Ctenopteryx, fins pectinate, as long as the body;Bathyteuthis, fins terminal, rudimentary; tentacular arms, filiform; abyssal.Rhynchoteuthis, tentacular arms united to form a beak-shaped appendage.Symplectoteuthis. Tracheloteuthis. Doridicus. Architeuthis; this is the largest of Cephalopoda, reaching 60 ft. in length including arms.
Fam. 7.Thysanoteuthidae. Arms enlarged, bearing two rows of suckers and filaments; fins triangular, extending whole length of body.Thysanoteuthis, Mediterranean.
Fam. 8.Onychoteuthidae. Fins terminal; tentacular arms long; suckers with hooks.Onychoteuthis, hook-bearing suckers on tentacular arms only.Enoploteuthis, hook-bearing suckers on all the arms.Veranya, body very short, tentacular arms atrophied in the adult, Mediterranean.Chaunoteuthis, body elongated, tentacular arms atrophied.Pterygioteuthis. Ancistroteuthis. Abralia. Teleoteuthis. Lepidoteuthis.
Fam. 9.Gonatidae. Body elongated; fins terminal; radula with only two lateral teeth.Gonatus.
Fam. 10.Cheiroteuthidae. Tentacular arms long, not retractile; resisting apparatus well developed.Cheiroteuthis, suckers along the whole length of the tentacular arms.Doratopsis, body very long and slender with aboral spine, dorsal arms very short.Histioteuthis, six dorsal arms united by membrane, photogenous organs present.Histiopsis, membrane of dorsal arms only half-way up the arms, photogenous organs present.Calliteuthis, no brachial membrane, photogenous organs present.Grimalditeuthis, two fins on each bide, no tentacular arms.
Fam. 11.Cranchiidae. Eight normal arms, very short; eyes prominent; fins small and terminal.Cranchia, body short, purse-shaped, normal arms short, fins entirely aboral.Loligopsis, body elongated, conical, tentacular arms slender.Leachia, tentacular arms absent, funnel without a valve.Taonius, body elongated, normal arms, rather short, eyes pedunculated.
A,Pinnoctopus cordiformis, Quoy and Gain (from New Zealand).
B,Tremoctopus violaceus, Ver. (from the Mediterranean).
C,Cranchia scabra, Owen (from the Atlantic Ocean; one of the Decapoda).
D,Cirrhoteuthis Mulleri, Esch. (from the Greenland coast).
Tribe 2.Myopsida.—No aperture in the cornea. Left oviduct only developed in female. Internal shell without a distinct phragmacone, calcified or simply chitinous.
Fam. 1.Sepiidae. Body wide and flat; fins narrow, extending the whole length of the body; shell calcareous and laminated.Belosepia, a rudiment of rostrum and phragmacone present in shell, Eocene.Sepia, shell with a rostrum, British.Sepiella, shell without a rostrum.Fam. 2.Sepiolidae. Body short, rounded at the aboral end; fins rounded, inserted in middle of body-length; shell chitinous, small or absent.Sepiola, head united to mantle dorsally, British.Rossia, head not united to mantle, British.StoloteuthisandInioteuthis, without shell.Heteroteuthis. Euprymna.Fam. 3.Idiosepiidae. Body elongated, with rudimentary terminal fins; internal shell almost lost.Idiosepius, 1.5 cm. long, Indian Ocean.Fam. 4.Sepiadariidae. Body short; mantle united to head dorsally; no shell.Sepiadarium, Pacific Ocean.Sepioloidea, Australian.Fam. 5.Loliginidae. Body elongated and conical; fins extending forward beyond the middle of body-length; shell chitinous, well developed.Loligo, fins triangular, aboral, British.Sepioteuthis, fins rounded, extending along whole of body-length.Loliolus. Loliguncula.The following fossil genera, known only by their gladius and ink-sac, have been placed nearLoligo:—Teuthopsis, BeloteuthisandGeoteuthis, Lias;Phylloteuthis, Cretaceous;Plesioteuthis, Jurassic and Cretaceous.
Fam. 1.Sepiidae. Body wide and flat; fins narrow, extending the whole length of the body; shell calcareous and laminated.Belosepia, a rudiment of rostrum and phragmacone present in shell, Eocene.Sepia, shell with a rostrum, British.Sepiella, shell without a rostrum.
Fam. 2.Sepiolidae. Body short, rounded at the aboral end; fins rounded, inserted in middle of body-length; shell chitinous, small or absent.Sepiola, head united to mantle dorsally, British.Rossia, head not united to mantle, British.StoloteuthisandInioteuthis, without shell.Heteroteuthis. Euprymna.
Fam. 3.Idiosepiidae. Body elongated, with rudimentary terminal fins; internal shell almost lost.Idiosepius, 1.5 cm. long, Indian Ocean.
Fam. 4.Sepiadariidae. Body short; mantle united to head dorsally; no shell.Sepiadarium, Pacific Ocean.Sepioloidea, Australian.
Fam. 5.Loliginidae. Body elongated and conical; fins extending forward beyond the middle of body-length; shell chitinous, well developed.Loligo, fins triangular, aboral, British.Sepioteuthis, fins rounded, extending along whole of body-length.Loliolus. Loliguncula.The following fossil genera, known only by their gladius and ink-sac, have been placed nearLoligo:—Teuthopsis, BeloteuthisandGeoteuthis, Lias;Phylloteuthis, Cretaceous;Plesioteuthis, Jurassic and Cretaceous.
Suborder 2. Octopoda.—Only four pairs of arms, all similar and longer than the body. Body short and rounded aborally. Suckers sessile. Heart not contained in coelom. No nidamentary glands.
Tribe I.Leioglossa.—No radula. Arms united by a complete membrane. Fins on sides of body.
Fam.Cirrhoteuthidae. Tentacular filaments on either side of the suckers.Cirrhoteuthis, pallial sac prominent, fins large, pelagic.Opisthoteuthis, body flattened, with small fins, deep-sea.Vampyroteuthis, four fins.Palaeoctopus, fossil, Cretaceous.
Fam.Cirrhoteuthidae. Tentacular filaments on either side of the suckers.Cirrhoteuthis, pallial sac prominent, fins large, pelagic.Opisthoteuthis, body flattened, with small fins, deep-sea.Vampyroteuthis, four fins.Palaeoctopus, fossil, Cretaceous.
Tribe 2.Trachyglossa.—Radula present. No fins.
Fam. 1.Amphitretidae. Arms united by membrane; funnel attached to mantle, dividing the pallial aperture into two.Amphitretus, pelagic.Fam. 2.Alloposidae. All arms united by membrane; mantle joined to head by dorsal band and two lateral commissures.Alloposus, pelagic.Fam. 3.Octopodidae. Arms long and equal, without membrane; hectocotylus not autotomous. No cephalic aquiferous pores.Octopus, two rows of suckers on each arm, British.Eledone, single row of suckers on each arm.Scaeurgus. Pinnoctopus. Cistopus. Japetella.Fam. 4.Philonexidae. Hectocotylus autotomous; arms unequal in size; aquiferous pores on head and funnel.Tremoctopus, two dorsal pairs of arms united by membrane.Ocythoë, without interbrachial membrane.Fam. 5.Argonautidae. Hectocotylus autotomous; no interbrachial membrane; extremities of dorsal arms in female expanded and secreting a shell; males very small, without shell.Argonauta.
Fam. 1.Amphitretidae. Arms united by membrane; funnel attached to mantle, dividing the pallial aperture into two.Amphitretus, pelagic.
Fam. 2.Alloposidae. All arms united by membrane; mantle joined to head by dorsal band and two lateral commissures.Alloposus, pelagic.
Fam. 3.Octopodidae. Arms long and equal, without membrane; hectocotylus not autotomous. No cephalic aquiferous pores.Octopus, two rows of suckers on each arm, British.Eledone, single row of suckers on each arm.Scaeurgus. Pinnoctopus. Cistopus. Japetella.
Fam. 4.Philonexidae. Hectocotylus autotomous; arms unequal in size; aquiferous pores on head and funnel.Tremoctopus, two dorsal pairs of arms united by membrane.Ocythoë, without interbrachial membrane.
Fam. 5.Argonautidae. Hectocotylus autotomous; no interbrachial membrane; extremities of dorsal arms in female expanded and secreting a shell; males very small, without shell.Argonauta.
Literature.—Use has been freely made above of the article by E. Ray Lankester, onMollusca, in the 9th edition of this Encyclopedia. For the chief modern works, see Bashford Dean, “Notes on Living Nautilus,”Amer. Nat.xxxv., 1901; Arthur Willey, “Contribution to the Natural History of the Pearly Nautilus,” A. Willey’sZoological Results, pt. vi. (1902); Foord,Cat. Fossil Cephalopoda in British Museum; Alpheus Hyatt, “Fossil Cephalopods of the Museum of Comp. Zoology,”Bull. Mus. Comp. Zool.(Cambridge, U.S., 1868); Jalta, “I Cefalopodi viventi nel golfo di Napoli,”Fauna und Flora des Golfes von Neapel, xxiii. (1896); Joubin, “Céphalopodes de l’atlantique nord,” “Céph. de la Princesse Alice,”Camp. sci. Albert Ierde Monaco, ix. (1895), xxii. (1900); Paul Pelseneer, “Mollusca,” in theTreatise on Zoology, edited by E. Ray Lankester.