fig94Fig. 94.—A longitudinal section through the anterior end ofEchinorhynchus haerucaRud. (From Hamann.)a, The proboscis not fully expanded;b, proboscis-sheath;c, retractor muscles of the proboscis;d, cerebral ganglion;e, retinaculum enclosing a nerve;f, one of the retractors of the sheath;g, a lemniscus;h, one of the spaces in the sub-cuticular tissue;i, longitudinal muscular layer;j, circular muscular layer;k, line of division between the sub-cuticular tissue of the trunk and that of the proboscis with the lemnisci.
Fig. 94.—A longitudinal section through the anterior end ofEchinorhynchus haerucaRud. (From Hamann.)a, The proboscis not fully expanded;b, proboscis-sheath;c, retractor muscles of the proboscis;d, cerebral ganglion;e, retinaculum enclosing a nerve;f, one of the retractors of the sheath;g, a lemniscus;h, one of the spaces in the sub-cuticular tissue;i, longitudinal muscular layer;j, circular muscular layer;k, line of division between the sub-cuticular tissue of the trunk and that of the proboscis with the lemnisci.
Fig. 94.—A longitudinal section through the anterior end ofEchinorhynchus haerucaRud. (From Hamann.)a, The proboscis not fully expanded;b, proboscis-sheath;c, retractor muscles of the proboscis;d, cerebral ganglion;e, retinaculum enclosing a nerve;f, one of the retractors of the sheath;g, a lemniscus;h, one of the spaces in the sub-cuticular tissue;i, longitudinal muscular layer;j, circular muscular layer;k, line of division between the sub-cuticular tissue of the trunk and that of the proboscis with the lemnisci.
All the spaces in the skin of the proboscis open ultimately into a circular canal situated round its base; on each side the canal opens into a sac-like structure which extends through the body-cavity towards the posterior end of the animal. These two lateral diverticula are termed thelemnisci. They have always attracted considerable attention from the workers at the group, and numerous functions have from time to time been attributed to them. They are more or less hollow, and their walls consist of sub-cuticular tissue surrounded with a scanty muscular coat; they contain the same fluid as the lacunae of the skin of the proboscis, with which they are placed in communication by means of the circular canal; and it seems most probable that, as Hamann[216]suggests, they act as reservoirs into which the lacunar fluid retires when the proboscis is retracted,and which, by means of the contractions of their muscular coat, force the fluid into the lacunae when the proboscis is everted, and thus aid in its protrusion.
The parasitic habits ofEchinorhynchushave had a deeper influence on the structure of the body than is the case with the Nematoda. All traces of an alimentary canal have disappeared, and the animals live entirely by the imbibition through the skin of the already elaborated fluids of their hosts. The power of absorbing fluids is shown by the fact that they swell up and become tense when placed in fresh water.
Until recently no definite excretory organs had been recognised, and the function of excreting the nitrogenous matter was by some assigned to the lemnisci. In 1893 Kaiser[217]described inG. gigastwo organs which he called nephridia, placed dorsally to the ducts of the male and female reproductive organs. Each nephridium, which somewhat resembles a cauliflower, consists of a stalk or duct, opening at one end into the reproductive ducts, and at the other branching and breaking up into a number of secondary and tertiary twigs. The end of each twig is closed by a membrane pierced with a number of most minute pores, by means of which it communicates with the body-cavity; on the inner side the membrane bears a number of long cilia, which keep up an active flickering. The presence of these cilia is interesting, as elsewhere they are unknown throughout the Nemathelminthes.
fig95Fig. 95.—A, A longitudinal section through the terminal twigs of the nephridium ofGigantorhynchus gigas. (From J. E. Kaiser.) Highly magnified.a, Nucleus.B, A terminal twig more highly magnified;b, the porous membrane.
Fig. 95.—A, A longitudinal section through the terminal twigs of the nephridium ofGigantorhynchus gigas. (From J. E. Kaiser.) Highly magnified.a, Nucleus.B, A terminal twig more highly magnified;b, the porous membrane.
Fig. 95.—A, A longitudinal section through the terminal twigs of the nephridium ofGigantorhynchus gigas. (From J. E. Kaiser.) Highly magnified.a, Nucleus.B, A terminal twig more highly magnified;b, the porous membrane.
The nervous system consists of a central ganglion situated in the proboscis sheath; it is oval and flattened in shape. The ganglion gives off nerves to the proboscis, and two main trunks which pierce the proboscis-sheath and run backward surrounded by a cluster of muscle-fibres, the whole being termed theretinaculum; in the male they are in connexion with a special genital ganglion which lies near the ductus ejaculatorius.
With the exception of certain sensory papillae in the neighbourhood of the male genital orifice, and of three similar papillae mentioned by Kaiser on the proboscis, the Acanthocephala are devoid of sense organs.
The Acanthocephala are dioecious; their generative organs are developed in connexion with theligament, a cord-like structure which arises between the inner and outer layer of the hinder end of the proboscis sheath and traverses the body-cavity, ending posteriorly in connexion with the genital ducts. The testes lie in this ligament; they are paired oval bodies which open each into a vas deferens. The vasa deferentia each bear three lateral diverticula, the vesiculae seminales; and three pairs of cement glands pour their secretion into a duct which opens into the vasa deferentia; the latter unite and open by a penis which is withdrawn into a genital bursa, but is capable of being extruded.
fig96Fig. 96.—An optical section through a maleNeorhynchus clavaecepsZed. (From Hamann.)a, Proboscis;b, proboscis sheath;c, retractor of the proboscis;d, cerebral ganglion;f,f, retractors of the proboscis sheath;g,g, lemnisci, each with two giant nuclei;h, space in sub-cuticular layer of the skin;l, ligament;m,m, testes;o, glands on vas deferens;p, giant nucleus in skin;q, opening of vas deferens.
Fig. 96.—An optical section through a maleNeorhynchus clavaecepsZed. (From Hamann.)a, Proboscis;b, proboscis sheath;c, retractor of the proboscis;d, cerebral ganglion;f,f, retractors of the proboscis sheath;g,g, lemnisci, each with two giant nuclei;h, space in sub-cuticular layer of the skin;l, ligament;m,m, testes;o, glands on vas deferens;p, giant nucleus in skin;q, opening of vas deferens.
Fig. 96.—An optical section through a maleNeorhynchus clavaecepsZed. (From Hamann.)a, Proboscis;b, proboscis sheath;c, retractor of the proboscis;d, cerebral ganglion;f,f, retractors of the proboscis sheath;g,g, lemnisci, each with two giant nuclei;h, space in sub-cuticular layer of the skin;l, ligament;m,m, testes;o, glands on vas deferens;p, giant nucleus in skin;q, opening of vas deferens.
The two ovaries are formed in the ligament of the female in a corresponding position to that occupied by the testes in the male, but at an early stage they break down into packets of cells, of which those of the peripheral layer develop into ova at the cost of the central cells, which serve them as a food supply. As these masses grow and increase in number they rupture the walls of the ligament, and escape into the body-cavity, in which they float. The ova arefertilised whilst floating in the fluid of the body-cavity. The eggs segment and the embryo is formed whilst still in the body of the mother.
The embryos escape by means of a complicated apparatus the details of which vary in the different species, but which, like many of the organs in these animals, consists of very few cells with very large nuclei. This apparatus consists of three parts: the bell, the uterus, and the oviduct. The bell is a large funnel-shaped structure, which opens into the body-cavity, and is connected with the end of the ligament; near its lower end, where it is continuous with the uterus, is a second smaller opening situated dorsally. By the contraction and expansion of its lips the oval embryos are swallowed and pass on through the uterus to the oviduct, which opens at the posterior end of the body. If the bell takes in any of the less mature eggs which are spherical in shape, they are passed back into the body-cavity through the above-mentioned dorsal opening, and the same orifice permits the passage of the spermatozoa even when the bell is full of embryos.
fig97Fig. 97.—An egg ofEchinorhynchus acusRud. surrounded by three egg-shells. Highly magnified. The egg has segmented, and the cells are differentiated intoa, the entoblast, andb, the ectoblast;c, spines. (From Hamann.)
Fig. 97.—An egg ofEchinorhynchus acusRud. surrounded by three egg-shells. Highly magnified. The egg has segmented, and the cells are differentiated intoa, the entoblast, andb, the ectoblast;c, spines. (From Hamann.)
Fig. 97.—An egg ofEchinorhynchus acusRud. surrounded by three egg-shells. Highly magnified. The egg has segmented, and the cells are differentiated intoa, the entoblast, andb, the ectoblast;c, spines. (From Hamann.)
Embryology.—After fertilisation the egg surrounds itself with several egg-shells, three of which are usually distinguished; the embryo is already far advanced in its development by the time it leaves the body of the mother and passes out into the alimentary canal of the Vertebrate host. It leaves the body of this second host with the faeces, and is eaten by the first or larval host, usually a small Crustacean or water-insect, but in some cases a fish, within whose alimentary canal it casts its membranes andbecomes actively mobile. By means of a ring of hooks developed round the anterior end it bores its way through the wall of the alimentary canal, and after some time—three weeks inE. proteus—comes to rest in the body-cavity of its host. By this time most of the organs of the adult, with the exception of the reproductive glands, are already well established; the latter only attain maturity when the first host is eaten by the second, and the larvae find themselves in the intestine of a Vertebrate.
fig98Fig. 98.—A, A larvalEchinorhynchus proteusWestrumb. further developed than in Fig. 97. Highly magnified. The entoblast has developed inside it the proboscisa;b,b, the giant nuclei of the ectoblast.B, The entoblast at a more advanced stage, the ectoblast is not shown. The outermost layer of cells will form the muscles of the body-wall; the body-cavity has appeared;a, proboscis;b, cerebral ganglion;c, body-cavity;d,d, the testes beginning to appear in the ligament;e, cells which will form the generative ducts.
Fig. 98.—A, A larvalEchinorhynchus proteusWestrumb. further developed than in Fig. 97. Highly magnified. The entoblast has developed inside it the proboscisa;b,b, the giant nuclei of the ectoblast.B, The entoblast at a more advanced stage, the ectoblast is not shown. The outermost layer of cells will form the muscles of the body-wall; the body-cavity has appeared;a, proboscis;b, cerebral ganglion;c, body-cavity;d,d, the testes beginning to appear in the ligament;e, cells which will form the generative ducts.
Fig. 98.—A, A larvalEchinorhynchus proteusWestrumb. further developed than in Fig. 97. Highly magnified. The entoblast has developed inside it the proboscisa;b,b, the giant nuclei of the ectoblast.B, The entoblast at a more advanced stage, the ectoblast is not shown. The outermost layer of cells will form the muscles of the body-wall; the body-cavity has appeared;a, proboscis;b, cerebral ganglion;c, body-cavity;d,d, the testes beginning to appear in the ligament;e, cells which will form the generative ducts.
Some of the details of the development are very remarkable, and a short account of them may be given. The segmentation of the egg is unequal; it results in the formation of a central biscuit-shaped mass of small cells and a peripheral mass of larger cells; the former is called by Hamann[218]the entoblast, the latter the ectoblast. From the entoblast arise all the organs of the body but the sub-cuticle and the associated lemnisci, which are formed from the ectoblast. The latter has a remarkable history; the cells begin to break down and lose their outlines, whilst their nuclei fuse together and form a small number of giant nuclei, which lie scattered throughout the syncytium thus formed. The syncytium surrounds the entoblast on all sides; by this time the anteriorly-placed hooks have appeared; inE. proteusthereare ten of these, but the number is not the same in all species. The syncytium is in a fluid state, with a few gigantic nuclei floating in it; these now lose their spherical shape, and throwing out processes become amoeboid; in this way they bud off small portions of their substance, and from these the oval nuclei of the sub-cuticle and the lemnisci arise. The rest of the syncytium hardens into the fibrillar matrix of the sub-cuticle, leaving, however, scattered spaces which form the sub-cuticular sinuses of the adult. An interesting feature ofN. clavaecepsandArhynchus hemignathiis that the skin of the adult retains the larval features, and it and the lemnisci consist of a syncytium with a very few giant nuclei scattered through it. Hamann counted only eight in the skin and two in each lemniscus in the example figured on p.178.
fig99Fig. 99.—A, The larva ofEchinorhynchus proteusfrom the body-cavity ofPhoxinus laevis, with the proboscis retracted and the whole still enclosed in a capsule.B, A section through the same;a, the invaginated proboscis;b, proboscis sheath;c, beginning of the neck;d, lemniscus. Highly magnified. (Both from Hamann.)
Fig. 99.—A, The larva ofEchinorhynchus proteusfrom the body-cavity ofPhoxinus laevis, with the proboscis retracted and the whole still enclosed in a capsule.B, A section through the same;a, the invaginated proboscis;b, proboscis sheath;c, beginning of the neck;d, lemniscus. Highly magnified. (Both from Hamann.)
Fig. 99.—A, The larva ofEchinorhynchus proteusfrom the body-cavity ofPhoxinus laevis, with the proboscis retracted and the whole still enclosed in a capsule.B, A section through the same;a, the invaginated proboscis;b, proboscis sheath;c, beginning of the neck;d, lemniscus. Highly magnified. (Both from Hamann.)
The whole of the rest of the body is formed by the entoblast. Within the latter a circular split arises which separates a single layer of outermost cells from an axial strand of many cells (Fig. 98, B). The split is the future body-cavity; the axial strand forms the proboscis, its sheath, the cerebral ganglion, muscles, etc., and the ligament with the contained generative organs; the outermost layer of cells forms the muscular lining to the skin. It is interesting to note that these cells destined to become muscle-fibres are at first arranged as a single layer of cubical epithelial cells lining the body-cavity; most of them become circular muscle-fibres, but a few are pushed inwards so as to lie next the body-cavity, and these become the longitudinal fibres.
Classification.—Until recently the Acanthocephala were supposed to include but one genus,Echinorhynchus, with several hundred species, but Hamann[219]has pointed out that these speciespresent differences which enabled him to divide the group into three families, each with a corresponding genus. To these I have ventured to add a fourth family, to include a remarkable species,Arhynchus hemignathi, described below. The characters of the first three families in the account given below are taken from Hamann's paper.
fig100Fig. 100.—Fully formed larva ofEchinorhynchus proteusfrom the body-cavity ofPhoxinus laevis. (From Hamann.) Highly magnified.a, Proboscis;b, bulla;c, neck;d, trunk;e,e, lemnisci.
Fig. 100.—Fully formed larva ofEchinorhynchus proteusfrom the body-cavity ofPhoxinus laevis. (From Hamann.) Highly magnified.a, Proboscis;b, bulla;c, neck;d, trunk;e,e, lemnisci.
Fig. 100.—Fully formed larva ofEchinorhynchus proteusfrom the body-cavity ofPhoxinus laevis. (From Hamann.) Highly magnified.a, Proboscis;b, bulla;c, neck;d, trunk;e,e, lemnisci.
Family I. Echinorhynchidae.—The body is elongated and smooth. The proboscis-sheath has a double wall, and the proboscis is invaginated into it. The central nerve-ganglion lies in the middle line, as a rule on the posterior blind end of the proboscis-sheath. The papillae which bear the hooks are only covered with a chitinous cap at their apex, and the hooks have a process below. This family is by far the largest; a few species only can be mentioned.Echinorhynchus proteuslives in its mature form in fishes; the young forms, up to a centimetre in length, are found living freely in the intestine of numerous fresh-water fishes. Those found inGobio fluviatilis, the gudgeon;Leuciscus virgo;Lota vulgaris, the burbot or eel-pout; young trout;Thymallus vulgaris, the grayling, seldom surpass this size, but those found inAcerina cernua, the pope fish; inAbramis bipunctatus; inEsox lucius,the pike, and in older trout, attain or surpass double the length. As the parasites grow older they bury their proboscis and neck in the wall of the intestine, the inner surface of which is studded with the orange-coloured bodies of the parasites. The proboscis is so deeply sunk in the wall of the alimentary canal as to form a papilla on its outer surface (Fig. 92). The larvae ofE. proteusare found in the body-cavity ofGammarus pulex, one of the Amphipod Crustacea, and also in the same position in numerous fresh-water fishes; they must have passed into this first host by themouth and alimentary canal. If the liver of an infested minnow,Leuciscus phoxinus, be examined, it will be found to contain on its surface numerous spherical or egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these contain the larval forms of the parasite. They develop into the adult form when the first host is eaten by a carnivorous fish, but a complication may take place when the larval form is found inGammarus, as the latter, the first host, may be eaten by a fish (intermediate host) in which the larva does not become mature, and only develops sexual organs when eaten by a carnivorous fish (second host). The larval form is also found inNemachilus barbatulus,Gobio fluviatilis, and the sticklebacksGasterosteus aculeatusandG. pungitius.
E. clavulaDuj. is found inSalmo fario,Abramis brama,Cyprinus carpio,Gobius niger,Lepadogaster gouanii, etc.;E. linstowiHam. inLeuciscus idus,Abramis ballerus,Abramis bipunctatus, andAcipenser huso;E. lutziiHam. was found by Dr. Lutz in Brazil in the intestine ofBufo agua;E. angustatusRud. occurs in such numbers in the perch,Perca fluviatilis, as to almost occlude the lumen of the intestine, and one out of every three or four fish in certain districts is infested by it. It is also found in the pike,Esox lucius, and the barbel,Barbus vulgaris. The first or larval host of this species is the IsopodAsellus aquaticus.E. moniliformisBrews. is stated to attain maturity in the human intestine. Except for the fact thatG. gigashas once been observed in the same place, this is the only human parasite amongst the Acanthocephala. Its normal second hosts areMus decumanusandMyoxus quercinus, and its first or larval host, the larvae of the beetleBlaps mucronata.E. porrigensRud. is found in considerable numbers in the small intestine of a fin-whale (Balaenoptera sibbaldii), andE. strumosusRud., in the small intestine of a seal (Phoca vitulina), and in the body-cavity of the angler fish (Lophius piscatorius).E. acusis common in the whiting,Gadus merlangus.
Family II. Gigantorhynchidae.—Large forms with ringed, flattened, andTaenia-like bodies. The hook-papillae are covered all over with transparent chitinous sheaths with two root-like processes. The proboscis-sheath is muscular and without a lumen. The central nervous system is excentrically placed below the middle of the so-called sheath. The lemnisci are long twisted tubes with a central canal.
Hamann places three species in this family:Gigantorhynchus echinodiscus,G. spira, andG. taenioides; but as he points out thatE. gigasresembles these in its more important structural features, it seems advisable to include it here under the nameG. gigas. The members of the first family often present a transversely ringed appearance after death, but the Gigantorhynchidae are ringed when alive, and the circular canals in the skin show a certain regularity, being arranged one between each two rings. There is no lumen in the proboscis-sheath, which is not attached to the boundary between the proboscis and the trunk, but to the inner surface of the proboscis, and the whole can be retracted within the anterior portion of the body, which is invaginable. There are always eight cement-glands, and other differences exist in the musculature, hooks, and position of the nervous system.
G. gigasoccurs in the adult state in the small intestine of swine; in Europe its first or larval host is believed to be the grubs ofMelolontha vulgarisandCetonia aurata, but these beetles are absent from America, though the parasite infests American hogs. Stiles[220]has recently made some experiments which tend to show that in the United States the source of infection is some species of the beetleLachnosterna, and he has succeeded in infecting the grub ofL. arcuataby feeding it on the eggs of the parasite; from one larva he took 300 parasites six weeks after feeding it.L. arcuatais, likeM. vulgaris, phytophagous, but the grubs of both the beetles are fond of frequenting manure heaps and patches of dung, and thus are much exposed to the dangers of infection.
G. echinodiscusinhabits the intestine of ant-eaters, having been found inMyrmecophaga jubataandCycloturus didactylus.G. spiralives in the king vultureSarcorhampus papa, andG. taenioidesinDicholophus cristatus, a species of Cariama.
Family III. Neorhynchidae.—Sexual maturity is reached in the larval stage. The proboscis-sheath has a single wall. A few giant nuclei only are found in the sub-cuticle and in the lemnisci. The circular muscle layer is very simply developed. The longitudinal muscle-cells are only present in certain places.
This family includes two species,Neorhynchus clavaecepsandN. agilis, which afford interesting examples of paedogenesis. The sub-cuticle and the lemnisci are dominated by a few giantnuclei, which remain in the embryonic state and do not break up into numerous nuclei as in other forms. The musculature is but little developed and the longitudinal sheath hardly exists. The proboscis-sheath consists of a simple muscular layer, and the short proboscis has few hooks and presents an embryonic appearance.
The sexually-mature form lives in the carp,Cyprinus carpio; the larval form is found, according to Villot,[221]encysted in the fat bodies of the larva ofSialis lutaria, one of the Neuroptera, and in the alimentary canal of the leechNephelis octocula, and successful experiments have been made in infecting some species of the water snailLimnaea.N. agilisoccurs inMugil auratusandM. cephalus.
Family IV. Arhynchidae.—Short forms with the body divided into three well-marked regions—head, collar, and trunk. The head is pitted, the collar smooth, and the trunk wrinkled, not annulated, in spirit specimens. There is no eversible introvert, and no introvert sheath and no hooks. The sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are long and coiled.[222]
This family resembles the Gigantorhynchidae in the length and curvature of its lemnisci, and the Neorhynchidae in the persistence of the embryonic condition of the nuclei in the sub-cuticle and the lemnisci; but in the shape of the body, its division into three well-marked regions, the absence of eversible proboscis, proboscis sheath, and hooks it stands alone, though it is nearer to the Neorhynchidae than to either of the other families.
The single speciesArhynchus hemignathiwas found attached to the skin around the anus of a Sandwich Island bird,Hemignathus proceros. The bird is a member of a family Drepanididae, which is entirely confined to the Sandwich Island group. Professor Newton tells me that it is probable that the "food ofHemignathusconsists entirely of insects which it finds in or under the bark of trees," hence it is probable that the second host of this parasite, if such exists, must be looked for amongst the Insecta.
CHAETOGNATHA
STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION TABLE OF IDENTIFICATION
At certain seasons and at certain times of the day the naturalist who is investigating the fauna of the surface of the sea is apt to find his tow-net crammed with innumerable transparent spindle-shaped animals, which by their number and the way in which they become entangled with rarer objects, often render useless the result of his labours. These animals belong to the class Chaetognatha, which includes three genera,Sagitta,Spadella, andKrohnia. Amongst them are divided about twenty species, some of which, however, are of doubtful value.
Anatomy.—The body of these animals is as transparent as crystal; it is elongated, and bears a resemblance to certain torpedos, except that the head forms a somewhat blunt termination to the spindle-shaped body. The tail bears a caudal fin, andSpadellaandKrohniahave a single pair, andSagittatwo pairs, of lateral fins; all of which are flattened horizontally.
The body is externally divisible into three regions—head, trunk, and tail—and these correspond with the arrangement of the internal organs.
fig101Fig. 101.—Sagitta bipunctata.a, Vesicula seminalis. × 4. (After Hertwig.)
Fig. 101.—Sagitta bipunctata.a, Vesicula seminalis. × 4. (After Hertwig.)
Fig. 101.—Sagitta bipunctata.a, Vesicula seminalis. × 4. (After Hertwig.)
The head is surrounded by a fold of skin, forming a hood,which is most prominent at the sides (Fig. 102,g); within the hood the head bears from two to four rows of short spines, and outside these a right and left row of sickle-shaped hooks, the free ends of which in a state of rest converge round the mouth, but when disturbed these hooks can be widely divaricated.
The cavity of the body, or coelom, is divided into three distinct chambers by the presence of two thin transverse walls or septa, one situated between the head and the trunk, the other between the trunk and the tail (Figs. 104, 105). In the head, this cavity is much reduced by the presence of special muscles which move the spines, hooks, etc.; and in the small species, such asSpadella cephaloptera, the other two cavities are almost entirely occupied by the digestive and reproductive organs[223]; but in the large species, e.g.Sagitta hexaptera, a considerable space is left between the internal organs and the skin, and this is occupied by a coelomic fluid. If the skin of one of these larger species be punctured the fluid escapes and the animal shrivels up. A longitudinal partition or mesentery, with numerous pores in it, runs through these spaces, dividing the body-cavity into a right and left half; in the region of the trunk this mesentery supports the alimentary canal.
In addition to certain muscles in the head, which move the hooks, etc., there is a muscular lining to the body-wall. This is divided into two dorsal and two ventral bands, much in the same way as in Nematodes. The muscle fibres are striated.
The mouth, situated either terminally—Spadella marioni[224]—or below the head, leads into a pharynx; this passes into an intestine lined by a single layer of ciliated cells with a few glandular ones intermingled. The intestine runs straight through the body without loop or coil, and opens by an anus situated at the junction of the trunk and the tail. In most cases the anus is ventral or on the lower surface, but Gourret asserts that inSpadella marioniit is on the upper surface.
There are no special respiratory, excretory, or circulatory organs, unless a glandular structure described by Gourret in the head ofSpadella marionibe a real kidney.
The nervous system consists of a supra-oesophageal ganglionor brain situated in the head, and of a ventral ganglion lying in the trunk; both these nerve centres are embedded in the epidermis, and are connected with one another by means of two stout peri-oesophageal nerves (Figs. 102, 104). The brain also gives off a pair of nerves to the eyes, another pair to the olfactory organ, and a pair which ultimately meet one another and so form a ring; on this are certain ganglia giving off nerves which supply the muscles of the head. Both the chief ganglia give off numerous nerves, which divide and split up into a network of fibres which permeate the whole skin.
The sense organs are comparatively simple. A pair of very small eyes lie in the skin of the head; they are of complex structure, and to some extent remind one of the simple eyes of certain Crustacea. Behind the eyes and also on the upper surface of the animal is an unpaired organ which is usually described as olfactory in function (Figs. 103, 105). This is a ring-shaped modification of the epidermis drawn out into different shapes in the various species. The modified epidermal cells bear long cilia. The remaining sensory organs found in the group consist of clumps of modified cells scattered in round groups over the surface of the body and of the fins. The central cells of each group bear long tactile hairs, and are surrounded by supporting cells.
fig102Fig. 102.—Head ofSagitta bipunctata.A, Dorsal view;B, ventral view. × about 33. (From Hertwig.)A,a, spines;b, nerves to lateral cephalic ganglia;c, hooks;d, cephalic ganglion;e, olfactory nerve;f, optic nerve;g, hood;h, commissure to ventral ganglion;j, olfactory organ:B,a,c, andgas inA;k, mouth.
Fig. 102.—Head ofSagitta bipunctata.A, Dorsal view;B, ventral view. × about 33. (From Hertwig.)A,a, spines;b, nerves to lateral cephalic ganglia;c, hooks;d, cephalic ganglion;e, olfactory nerve;f, optic nerve;g, hood;h, commissure to ventral ganglion;j, olfactory organ:B,a,c, andgas inA;k, mouth.
Fig. 102.—Head ofSagitta bipunctata.A, Dorsal view;B, ventral view. × about 33. (From Hertwig.)A,a, spines;b, nerves to lateral cephalic ganglia;c, hooks;d, cephalic ganglion;e, olfactory nerve;f, optic nerve;g, hood;h, commissure to ventral ganglion;j, olfactory organ:B,a,c, andgas inA;k, mouth.
The Chaetognatha are hermaphrodite, and carry the female organs in the trunk, the male in the tail. In a mature specimen the two ovaries occupy almost all the space in the trunk between the alimentary canal and the skin, and each is supported by a narrow lateral mesentery. The ovary is traversed by a oviduct which often contains spermatozoa; it is not clear how the eggs make their way into the oviduct, which seems to haveno internal opening and to act largely as a receptaculum seminis. The oviducts open externally on the upper side at the base of the lateral fin, close to the junction of the tail and the trunk.
The cavity of the tail is divided into two lateral chambers by the extension backward of the median vertical mesentery. In each of these a testis and a vas deferens are found. The testes are solid ridges formed by the growth of the lining cells of this part of the body-cavity; the cells mature into spermatozoa, which break off and float freely in the coelomic fluid. At the breeding season the whole tail may be crowded with masses of spermatozoa, which are kept in a more or less regular circulation by the ciliated cells lining the body-wall. The vas deferens opens internally into the space where the spermatozoa lie, and at the other end into a vesicula seminis, which opens to the exterior. The position of the latter structure varies, and is of some systematic value.
The eggs are laid in the water and as a rule float at the surface of the sea.Spadella cephalopterais, however, an exception to this rule, as it attaches its eggs by means of a gelatinous stalk to sea-weeds. The segmentation of the ovum is regular, and gives rise to a two-layered stage orgastrula, which opens by a pore, theblastopore. This does not, however, become the mouth, but closes up and the mouth arises at the opposite pole. Perhaps the most interesting feature of the development ofSagittais that the cells destined to form the reproductive organs separate from the other cells of the embryo at a very early date, whilst it is still in the gastrula stage. There is no larval form, but the young hatch out from the egg in a state resembling the adult in all respects but that of size.
fig103Fig. 103.—Spadella cephaloptera.Dorsal view. x 30. (From Hertwig.)a, Cephalic ganglion;b, commissure to ventral ganglion;c, olfactory organ;d, alimentary canal;e, ovary;f, oviduct;g, testis;h, vesicula seminalis.
Fig. 103.—Spadella cephaloptera.Dorsal view. x 30. (From Hertwig.)a, Cephalic ganglion;b, commissure to ventral ganglion;c, olfactory organ;d, alimentary canal;e, ovary;f, oviduct;g, testis;h, vesicula seminalis.
Fig. 103.—Spadella cephaloptera.Dorsal view. x 30. (From Hertwig.)a, Cephalic ganglion;b, commissure to ventral ganglion;c, olfactory organ;d, alimentary canal;e, ovary;f, oviduct;g, testis;h, vesicula seminalis.
Habits.—The Chaetognatha are essentially pelagic, andresemble many other creatures that dwell at the surface of the ocean in being almost completely transparent. Most species have been taken far out at sea, but some are perhaps rather more numerous near the coast, and one species,Spadella cephaloptera, is littoral. They swim by means of muscular movements of the whole body; the fins have no movement of their own, and seem to serve as balancers, and not as locomotory organs. Although usually found at the surface of the water, many species have been taken at considerable depths. Chun[225]states that they are found in countless numbers at depths of from 100 metres to 1300 metres. The commonest species at these depths areSagitta hexapteraandSagitta serratodentata.Sagitta bipunctata, according to the same authority, confines itself to the surface. Whether the change of depth is diurnal, or whether it has any relation to sexual maturity, or to any other cause, has not been satisfactorily determined.
The food of the Chaetognatha consists of floating diatoms, Infusoria, small larvae, and such Copepods asCalanus finmarchicus, and small Amphipods asPhoxus plumosus.[226]At times they also devour small larval or post-larval fishes, and owing to their incredible numbers, they doubtless do considerable damage to sea fisheries. It is also recorded that they eat one another, and specimens have been taken which have ingested the whole body of anotherSagittaexcept the head, which hangs out of the mouth of the eater, and gives it the appearance of a double-headed monster.[227]It has been said that they attack hydroid polypes, but here at any rate they do not have it all their own way. Masterman[228]has figured the apical group of five polypes ofObelia, three of which are engaged in ingesting as many youngSagitta.
They exist in incredible numbers; Grassi describes the surface of the sea at Messina on certain days as being literally covered with them, and they must form the food supply of numerous animals which prey upon the pelagic fauna. The immense number of individuals is probably accounted for to some extent by the fact that they lay eggs all the year round, and passthrough a very short and rapid development. They are not known to be phosphorescent.
Classification.—The features of the Chaetognatha which have most systematic value are the size of the adult, the relations of the length to the breadth, and of the three divisions to one another; the size, number, and position of the lateral fins, and of the hooks and spines on the head; the thickness of the epidermis, and the structure of the olfactory organ; and, finally, the form of the reproductive organs.
Strodtmann,[229]who gives the latest and most complete account of the species of Chaetognatha, arranges them under three genera, which he characterises as follows:—
(i.)SagittaSlabber.—Two pairs of lateral fins, two rows of spines on the head. The lateral thickening of the epidermis absent or insignificant.
Under this genus are included nine definite species and five others—S. gracilisVerrill,S. elegansVerrill,S. darwiniGrassi,S. dipterad'Orbigny, andS. tripterad'Orbigny—whose position, owing to the inadequacy of their description, is of doubtful validity.
fig104Fig. 104.—Sagitta hexaptera.Ventral view. × 4. (From Hertwig.)a, Mouth;b, hooks;c, anterior septum;d, alimentary canal;e, commissure from the brain to the ventral ganglion;f, ventral ganglion;g, ovary;h, oviduct;i, posterior septum;j, testis;k, vesicula seminalis.
Fig. 104.—Sagitta hexaptera.Ventral view. × 4. (From Hertwig.)a, Mouth;b, hooks;c, anterior septum;d, alimentary canal;e, commissure from the brain to the ventral ganglion;f, ventral ganglion;g, ovary;h, oviduct;i, posterior septum;j, testis;k, vesicula seminalis.
Fig. 104.—Sagitta hexaptera.Ventral view. × 4. (From Hertwig.)a, Mouth;b, hooks;c, anterior septum;d, alimentary canal;e, commissure from the brain to the ventral ganglion;f, ventral ganglion;g, ovary;h, oviduct;i, posterior septum;j, testis;k, vesicula seminalis.
The distribution of the other species may be mentioned.S. hexapterais the largest Chaetognath known, and reaches in the adult stage a length of 7 cm. It is very widely distributed, being found in practically all the temperate and warm seas, usually at the surface of the water, though at times it is found at a depth of one metre, or even deeper.S. lyra, Mediterranean, very rare.S. tricuspidata, widely distributed.S. magna, Mediterranean and Madeiran, living at the surface.S. bipunctata, the most frequently described form, smaller than the preceding species, 1-2cm. in length, widely distributed, and as a rule living near the coast line.S. serratodentata, Mediterranean.S. enflata, on the surface of the sea, Mediterranean and Madeiran.S. minima, a very small species, 1 cm. in length, Mediterranean.S. falcidens, Atlantic, off the coast of New Jersey.
(ii.)KrohniaLangerhans.—A single lateral fin extending on to both trunk and tail segment, no lateral epidermal extensions behind the head, only one row of spines on the head. Trunk longer than the tail.
Krohniahas but two species:K. hamataMöbius, with a length of 3-4 cm., found in the North Atlantic and at considerable depths, 200 to 300 fathoms; andK. subtilisGrassi, 1.5 cm. long, with an extraordinary slender body and a relatively large head, found at Messina, but very rare; as a rule only one specimen has been found at a time.
(iii.)SpadellaLangerhans.—A single pair of lateral fins; these are situated on the tail segment. Behind the head a thickening of the epidermis extends down each side of the body to the fin, or even farther. Two rows of spines on the head. Small animals, not longer than 1 cm.
fig105Fig. 105.—Spadella draco.Dorsal view. × 12. (From Hertwig.)a, Cephalic ganglion;b, commissure between the cephalic ganglion and the ventral;c, eye;d, olfactory organ;e, alimentary canal;f, ovary;g, oviduct (the line goes a little beyond the duct);h, testis;j, vesicula seminalis.
Fig. 105.—Spadella draco.Dorsal view. × 12. (From Hertwig.)a, Cephalic ganglion;b, commissure between the cephalic ganglion and the ventral;c, eye;d, olfactory organ;e, alimentary canal;f, ovary;g, oviduct (the line goes a little beyond the duct);h, testis;j, vesicula seminalis.
Fig. 105.—Spadella draco.Dorsal view. × 12. (From Hertwig.)a, Cephalic ganglion;b, commissure between the cephalic ganglion and the ventral;c, eye;d, olfactory organ;e, alimentary canal;f, ovary;g, oviduct (the line goes a little beyond the duct);h, testis;j, vesicula seminalis.
S. cephalopteraBusch is the smallest species of Chaetognatha, attaining at most a length of .5 cm. The body is not so transparent as in other species, and is of a yellowish colour. It has been found from the Orkney Islands to the Mediterranean. Strodtmann is of the opinion that the three speciesS. marianaLewes,S. batzianaGiard, andS. gallicaPagenstecher differ from the above-named only in size, or that their description is too indefinite to permit of accuratecharacterisation. He recognises three other distinct species:S. ponticaUljanin, from the Black Sea;S. marioniGourret, from the Gulf of Lyons; andS. dracoKrohn, Mediterranean and Madeiran, and from the Canaries.
Much confusion has been introduced into the classification of the Chaetognatha by Grassi,[230]who calls some—but not all—of what other writers termSagitta,Spadella, andvice versâ. The following table was compiled by Strodtmann,[231]but I have incorporated in it two species recently described from Amboyna by Béraneck,[232]and called by himSagitta bedotiandSpadella vougairespectively:—
CHAETOGNATHA
I. Two pairs of lateral fins; two rows of spines on the head; slender forms.(i.) Number of spines in posterior row greater than in anterior.a.Border of hooks smooth, their point not curved.α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—Sagitta lyra.β. A distinct interval between the two fins on each side.aa.Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.bb.Greatest length 1-2 cm.αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.b.Edge of hooks toothed and their point bent round; hooks 6-8; anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender; conspicuously projecting vesiculae seminales.—Sagitta serratodentata.(ii.) Number of the spines in posterior row smaller than in anterior.a.Anterior spines 3; posterior spine 1; hooks 8; length 3.5 cm.—Sagitta tricuspidata.b.Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1 cm.; tail ⅕ of the total length.—Sagitta magna.II. One pair of lateral fins lying on the trunk and tail; one row of spines; body slender; epidermis not thickened.(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young; 20-25 spines in a row; ovary reddish; length 3-4 cm.—Krohnia hamata.(ii.) Hooks 8, broad at their base but very sharply pointed; spines in a curved row, about 18, with a constriction below like the neck of a bottle; body thin; length 1-1.5 cm.—Krohnia subtilis.III. One pair of lateral fins, these lie on the tail; body relatively very broad in consequence of the thickening of the epidermis lying behind the head; two rows of spines; greatest length 1 cm.; tail and trunk usually the same length.(i.) A great extension of the epidermis behind the head, consisting of very large cells; amongst these, at the level of the ventral ganglion, lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10; anterior spines 6-8; posterior spines 12-18.—Spadella draco.(ii.) Lateral extension of the epidermis not so conspicuous, and the cells composing it smaller. Tactile organs in little depressions. Transverse as well as longitudinal muscles in the trunk. Adhesive cells on the ventral surface of the body. No interval between the lateral fins and the tail fin. Two papillae on the head-hood elongated into club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines 3-4; posterior spines 3-4.—Spadella cephaloptera.(iii.) Similar to the last-mentioned species, but the tail segment is larger than the trunk; in the above it is of the same size. No adhesive cells. The fins are covered with papillae, and with a number of serrated spines pointed at both ends.—Spadella pontica.(iv.) Tactile organs and adhesive cells are unmodified epidermal cells. Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis colourless. Lateral fins without rays. A pair of ganglia at the postero-lateral angle of the brain.—Spadella marioni.(v.) Tactile organs well developed on the head, trunk, and fins; tail segment a little shorter than the trunk. Body short, length 3-4 mm. Hooks 9; anterior spines 4-5, posterior spines 6-7.—Spadella vougai.
I. Two pairs of lateral fins; two rows of spines on the head; slender forms.
(i.) Number of spines in posterior row greater than in anterior.a.Border of hooks smooth, their point not curved.α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—Sagitta lyra.β. A distinct interval between the two fins on each side.aa.Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.bb.Greatest length 1-2 cm.αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.b.Edge of hooks toothed and their point bent round; hooks 6-8; anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender; conspicuously projecting vesiculae seminales.—Sagitta serratodentata.(ii.) Number of the spines in posterior row smaller than in anterior.a.Anterior spines 3; posterior spine 1; hooks 8; length 3.5 cm.—Sagitta tricuspidata.b.Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1 cm.; tail ⅕ of the total length.—Sagitta magna.
(i.) Number of spines in posterior row greater than in anterior.
a.Border of hooks smooth, their point not curved.α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—Sagitta lyra.β. A distinct interval between the two fins on each side.aa.Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.bb.Greatest length 1-2 cm.αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.b.Edge of hooks toothed and their point bent round; hooks 6-8; anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender; conspicuously projecting vesiculae seminales.—Sagitta serratodentata.
a.Border of hooks smooth, their point not curved.
α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—Sagitta lyra.β. A distinct interval between the two fins on each side.aa.Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.bb.Greatest length 1-2 cm.αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.
α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—Sagitta lyra.
β. A distinct interval between the two fins on each side.
aa.Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.bb.Greatest length 1-2 cm.αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.
aa.Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.
bb.Greatest length 1-2 cm.
αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.
αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.
ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.
γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.
δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.
εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.
b.Edge of hooks toothed and their point bent round; hooks 6-8; anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender; conspicuously projecting vesiculae seminales.—Sagitta serratodentata.
(ii.) Number of the spines in posterior row smaller than in anterior.
a.Anterior spines 3; posterior spine 1; hooks 8; length 3.5 cm.—Sagitta tricuspidata.b.Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1 cm.; tail ⅕ of the total length.—Sagitta magna.
a.Anterior spines 3; posterior spine 1; hooks 8; length 3.5 cm.—Sagitta tricuspidata.
b.Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1 cm.; tail ⅕ of the total length.—Sagitta magna.
II. One pair of lateral fins lying on the trunk and tail; one row of spines; body slender; epidermis not thickened.
(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young; 20-25 spines in a row; ovary reddish; length 3-4 cm.—Krohnia hamata.(ii.) Hooks 8, broad at their base but very sharply pointed; spines in a curved row, about 18, with a constriction below like the neck of a bottle; body thin; length 1-1.5 cm.—Krohnia subtilis.
(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young; 20-25 spines in a row; ovary reddish; length 3-4 cm.—Krohnia hamata.
(ii.) Hooks 8, broad at their base but very sharply pointed; spines in a curved row, about 18, with a constriction below like the neck of a bottle; body thin; length 1-1.5 cm.—Krohnia subtilis.
III. One pair of lateral fins, these lie on the tail; body relatively very broad in consequence of the thickening of the epidermis lying behind the head; two rows of spines; greatest length 1 cm.; tail and trunk usually the same length.
(i.) A great extension of the epidermis behind the head, consisting of very large cells; amongst these, at the level of the ventral ganglion, lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10; anterior spines 6-8; posterior spines 12-18.—Spadella draco.(ii.) Lateral extension of the epidermis not so conspicuous, and the cells composing it smaller. Tactile organs in little depressions. Transverse as well as longitudinal muscles in the trunk. Adhesive cells on the ventral surface of the body. No interval between the lateral fins and the tail fin. Two papillae on the head-hood elongated into club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines 3-4; posterior spines 3-4.—Spadella cephaloptera.(iii.) Similar to the last-mentioned species, but the tail segment is larger than the trunk; in the above it is of the same size. No adhesive cells. The fins are covered with papillae, and with a number of serrated spines pointed at both ends.—Spadella pontica.(iv.) Tactile organs and adhesive cells are unmodified epidermal cells. Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis colourless. Lateral fins without rays. A pair of ganglia at the postero-lateral angle of the brain.—Spadella marioni.(v.) Tactile organs well developed on the head, trunk, and fins; tail segment a little shorter than the trunk. Body short, length 3-4 mm. Hooks 9; anterior spines 4-5, posterior spines 6-7.—Spadella vougai.
(i.) A great extension of the epidermis behind the head, consisting of very large cells; amongst these, at the level of the ventral ganglion, lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10; anterior spines 6-8; posterior spines 12-18.—Spadella draco.
(ii.) Lateral extension of the epidermis not so conspicuous, and the cells composing it smaller. Tactile organs in little depressions. Transverse as well as longitudinal muscles in the trunk. Adhesive cells on the ventral surface of the body. No interval between the lateral fins and the tail fin. Two papillae on the head-hood elongated into club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines 3-4; posterior spines 3-4.—Spadella cephaloptera.
(iii.) Similar to the last-mentioned species, but the tail segment is larger than the trunk; in the above it is of the same size. No adhesive cells. The fins are covered with papillae, and with a number of serrated spines pointed at both ends.—Spadella pontica.
(iv.) Tactile organs and adhesive cells are unmodified epidermal cells. Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis colourless. Lateral fins without rays. A pair of ganglia at the postero-lateral angle of the brain.—Spadella marioni.
(v.) Tactile organs well developed on the head, trunk, and fins; tail segment a little shorter than the trunk. Body short, length 3-4 mm. Hooks 9; anterior spines 4-5, posterior spines 6-7.—Spadella vougai.