mm. The body is faintly ringed, and bears two rows of fine bristles on each

side. Owing to a curious torsion of the body through a right angle, the lateral bristles of the anterior third seem to be placed in the ventral and dorsal middle line. They are very easily broken off. The body is divided into a small anterior and a large posterior {169}chamber by a transverse septum placed a little way behind the head. The anterior chamber contains the brain and is lined by a definite epithelium, the posterior is not. The layers of the skin correspond with those of Nematodes or of _Gordius_, but the hypodermal cells show no cell outlines; still they are not so modified as in the former group. The hypodermis is thickened in the median dorsal and ventral line, and the single nerve-cord lies in the latter.

The alimentary canal is degenerate, as in _Gordius_. A mouth exists, but it is minute, and opens into a very fine tube lined with chitin, which pierces through the substance of a single elongated cell. This minute oesophagus, with its coextensive cell, reaches back to the transverse partition, but behind this a few other cells become associated with it, and ultimately the lumen of the alimentary canal is surrounded by four cells; but the number diminishes behind, and soon only two cells surround the tube at any one level, and the intestine dwindles away some little distance in front of the tail. There is no sign of an anus. A circumoesophageal nerve-ring exists, of which the ventral part is by far the larger (Fig. 87); it gives off a ventral nerve-cord, which swells posteriorly in the male into a large anal ganglion, far bigger than the brain, and larger in the male than in the female.

The testes consist of a dorsally placed sac, continuous behind with a vas deferens; this opens at the posterior end, which is pointed and slightly curved ventrally. The ovary is unknown; but females have been found with their body-cavity crammed with ova; these escape, like the spermatozoa, from a genital pore at the posterior end of the body.

CLASSIFICATION.—The separation of the Nematomorpha from the Nematoda depends mainly on the character of the nervous system, the absence of the lateral lines and of the dorsal line, the character of the contents of the body-cavity, and the character of the reproductive organs. In Gordiidae the latter are always placed dorsal to the intestine, and ovaries and testes open alike at the hinder end of the body. The importance of the differences in the organs just enumerated has been considered sufficient to justify the removal of the Gordiidae from the Nematoda, and the establishment of the special sub-Order Nematomorpha for their reception; and although _Nectonema_ has a dorsal line, and is in some other respects intermediate between the two groups, there can be little doubt that it is more closely allied to _Gordius_ than to any member {170}of the Nematoda, and it must therefore be placed with it in the Nematomorpha.

On the other hand, it ought to be mentioned that Camerano[210] found that the chief details of the fertilisation and development of the egg in _Gordius_ closely conform with what is known of the same processes in Nematodes, and he is of opinion that these resemblances are sufficiently important to justify the retention of the group among the Nematoda.

LIFE-HISTORY.—The life-history of _Gordius_ comprises four stages—the early development of the egg, the first larval form, the second larval form, and the sexually mature form. Both larval forms are parasitic, and during their life they are actively engaged in feeding; the free form, on the other hand, takes in no nourishment, and is exclusively engaged in reproduction.

Von Linstow[211] gives the following account of the life-history of _G. tolosanus_, a form which has been more fully worked out than any other. In the month of April numerous specimens of the beetle _Pterostichus niger_ were found floating on the surface of the ditches and small ponds in the fields surrounding Göttingen. Some were found dead or dying; others appeared quite healthy, and these were swimming actively, endeavouring to reach land. Within the abdomen of these beetles, in about 20 per cent of those collected, the second larval form of the _G. tolosanus_ was found. The longest larvae were 122 mm. in length, and very soft, partly snow-white and partly brown in colour; traces of the boring apparatus of the first larval form were still to be seen, but in other respects the larva only differed from the free form in the immaturity of its sexual organs. Besides the parasite hardly anything was to be found in the abdomen of the beetle, the larva having eaten up all trace of the fat body and the generative organs of its host. The larvae bored their way out of the body of the beetle and became adult animals.

It is rather difficult to say what brings these essentially {171}terrestrial beetles to the water, but von Linstow suggests that, as they live partly on snails, and at this time of year there are not many land-snails about, they may be in search of water-snails such as _Limnaea_. They may also be sometimes blown into the water by wind storms, but, whatever the cause is, their presence in water is essential for the continuance of the life of their parasites.

Once free in the water the _Gordius_ is soon sexually mature; the fertilisation takes place in April, and then the female may be seen twisting and writhing round the stems of water-plants and laying the long bead-like strands of eggs (Fig. 82). The first deposition observed by von Linstow took place on 14th April, the last on 2nd August, and the period of egg-laying for each female extended over four weeks. At first the eggs are snow-white, but within twenty-four hours they turn brown in colour.

The development of the first larva within the egg takes about a month. When it emerges from the egg-shell it is minute, .065 mm. long, ringed anteriorly, and provided with a protrusible and retractile boring apparatus consisting of three chitinous rods; round the base of this piercing proboscis is a double crown of papillae, each bearing a spine (Fig. 90).

This first larval form breaks through the egg-shell and sinks to the bottom of the water, where it moves about sluggishly and awaits the arrival of the right host in which to take up its abode. This host is the larva of the Alder-fly, _Sialis lutaria_ Lin. (_vide_ vol. v. p. 444), and into this it bores and comes to rest in the muscles or the fat body. It does not form distinct capsules. It remains in this larva during the following winter, and in the spring passes over into the imago _Sialis_. The complete insect frequents the small plants growing along the water's edge, and falls an easy prey to the predaceous beetle _Pt. niger_. The larva is eaten, and undergoing a change becomes the second larval form mentioned above. It remains in the body of the beetle during the second winter, and finally returns to the water {172}as the adult some eighteen or twenty months after it has been hatched from the egg.

From the above account of the life-history of _Gordius_ it will be seen that the chances of an egg reaching maturity are comparatively small, and to compensate for this a very large number of eggs are laid. In addition to the risk of the larvae not finding the right host at the right time, and of the first host not being eaten by the second, and the second not being drowned, there is the danger that the ditches and ponds in which the adults live may dry up, and, in fact, great numbers of worms perish by this taking place.

The sex of the adults may be told from their colour, the males being of a blackish brown, the females of a light clay brown; the former average 120 mm. in length, the latter 170 mm. The males are also more numerous, the proportion being seven to three. Camerano[213] has drawn attention to the fact that there is a certain polymorphism in size, form, and colour which is especially common amongst the males; dwarf forms with mature reproductive organs exist, and he is of opinion that these differences depend both on the size of the second host and on the duration of the parasitic life.

In addition to the larva of _Sialis lutaria_, the first larval stage has also been found in the larva of _Ephemera_, _Tanypus_, _Corethra_, and _Chironomus_; the second in _Carabus hortensis_ Fabr., _Procerus_ (_Carabus_) _coriaceus_ Linn., _Calathus fuscipes_ Goeze, _Molops elatus_ Fabr., several species of _Pterostichus_, and a number of other beetles. It is probable that its normal hosts are _S. lutaria_ and _Pt. niger_, but it is clear that it often comes to rest in other insects. The view that the Gordiidae have no special hosts, but may either pass the whole of their life-history within one and the same animal, or, on the other hand, may inhabit animals belonging to very different groups, is held by Villot, who has paid great attention to the subject. He finds the first larval form encysted in the walls of the alimentary canal in fishes, such {173}as _Leuciscus phoxinus_, the minnow, _Cobitis barbatula_, the loach, and _Petromyzon planeri_, the lamprey; in the larvae of Diptera, _Ephemera_, and beetles, in _Planorbis_ (a water snail), in _Enchytraeus_ (an Oligochaet); the second larval form in all kinds of insects, spiders, Crustacea, fish, frogs, birds (_Otis_), and in man, and these various habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes spéciaux." On the other hand, as von Linstow points out, it is contrary to our knowledge of parasites that a single species should develop equally well in the body of warm and cold-blooded Vertebrates and of Insects, and the explanation of the presence of the larvae in these various forms may either be that they belong to different species of _Gordius_ or, more probably, that they are accidentally present, having passed into their hosts with drinking water.

The number of species of _Gordius_ is large; over 100 are enumerated in the _Compendium der Helminthologie_,[214] the great majority of which inhabit insects.

The life-history of _Nectonema_ is practically unknown; the adults have been found swimming near the surface of the sea at two places only: Newport, R.I., and Wood's Holl, Mass., on the south coast of New England. It has been fished close to the shore, from the end of June to the beginning of October, when the tide is going out at evening and there is no moon. This seems to indicate that it avoids the light. When first caught the worms move actively about, coiling themselves into figures of eight and then uncoiling; at the same time there is a rhythmical movement caused by waves of muscular contraction passing down each side of the body alternately; by this kind of motion they make rapid and definite progress through the water.

It seems probable that the adult _Nectonema_ is preceded by one or more larval stages, and what appears to be a young form has {174}been obtained from the thoracic cavity of a prawn, _Palaemonetes_,[215] which has thus some claim to be regarded as the host of this species, but nothing is known about its early life-history.

SUB-ORDER III. ACANTHOCEPHALA.

The Acanthocephala, which form the third class of the Nemathelminthes, consists of but few genera; there are, however, numerous species of very different size, varying from 10 to 65 cm. long in the female _Gigantorhynchus_ (_Echinorhynchus_) _gigas_, to quite minute forms a few millimetres in length. The adult stage occurs in the alimentary canal of Vertebrates, as a rule in those which live in, or frequent water; the larvae are found in the bodies of certain Invertebrates, very frequently small Crustacea.

ANATOMY.—The body of the mature forms can usually be divided into three sections—the proboscis, the neck, and the trunk, but the middle region is not always discernible. The proboscis is armed with rings of hooks (Fig. 93) arranged in longitudinal rows; they are usually of two kinds, but in _E. proteus_ of three. They have a certain specific value, but not much stress can be laid on the number of rings, _e.g._ in _E. angustatus_ the number varies from eight to twenty-four. The recurved hooks serve to fasten the parasite very firmly to the tissues of the host. The proboscis is hollow and retractile; it can be withdrawn into the body by means of muscles attached internally to its tip. It does not, however, pass straight into the body-cavity, but is retracted into a special cavity—the proboscis sheath—with a double muscular wall. The proboscis sheath may perhaps be looked upon as a septum, such as is found in some of the Nematomorpha, dividing the body-cavity into two parts. It is inserted into the body-wall at the junction of the neck and trunk or of the proboscis and trunk. In addition to the muscles which withdraw the proboscis into its sheath, there are two retractors running from the {175}outside of the sheath to the body-wall; these serve to retract the whole sheath and its contents into the body-cavity of the trunk.

The structure of the skin is essentially like that of Nematodes, but the details are much more complicated. The whole body is covered by a thin cuticle secreted by the epidermis, which, as in the other groups, breaks down and forms a syncytium called the sub-cuticle. The minute fibrils which penetrate this layer are much more definitely arranged than in Nematodes; the largest of them run from without inwards, others run concentrically round the body. Large oval or spherical nuclei are scattered in the sub-cuticle, which is further honeycombed by a number of lacunae or spaces which are described below.

Within the sub-cuticular layer is found a sheath of circularly-arranged muscle-fibres, and within this again a sheath of longitudinal muscles which do not extend into the proboscis; this inner layer lines the body-cavity, there being no epithelium within it. In their minute structure the muscle-cells resemble those of Nematodes.

The canals in the sub-cuticle form a very curious system of anastomosing spaces, in which a clear fluid containing fat globules circulates. The extent to which the system is developed varies in different species, but in all there is a pair of longitudinal canals which are situated laterally, and which give off the subsidiary channels in their course. The above description applies to the lacunar spaces in the skin of the trunk; those of the proboscis are quite distinct, and there is no communication between the two sets of spaces; in fact, the sub-cuticle in which the lacunae are formed is not continuous across the line of junction of the proboscis and the neck, or, when the latter is absent, of the proboscis and the trunk, but it is interrupted by the ingrowth of a thin ring of cuticle which reaches down to the muscular layers (Fig. 94).

{176}[Illustration: FIG. 94.—A longitudinal section through the anterior end of _Echinorhynchus haeruca_ Rud. (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 the _lemnisci_. 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, {177}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 of _Echinorhynchus_ have 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 in _G. gigas_ two 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.

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 the {178}_retinaculum_; 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 the _ligament_, 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.

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 are {179}fertilised 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.

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 and {180}becomes 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 in _E. 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.

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; in _E. proteus_ there {181}are 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 of _N. clavaeceps_ and _Arhynchus hemignathi_ is 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.

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 species {182}present 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.

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 proteus_ lives 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 in _Gobio 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 in _Acerina cernua_, the pope fish; in _Abramis bipunctatus_; in _Esox 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 of _E. proteus_ are found in the body-cavity of _Gammarus 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 the {183}mouth 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 in _Gammarus_, 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 in _Nemachilus barbatulus_, _Gobio fluviatilis_, and the sticklebacks _Gasterosteus aculeatus_ and _G. pungitius_.

_E. clavula_ Duj. is found in _Salmo fario_, _Abramis brama_, _Cyprinus carpio_, _Gobius niger_, _Lepadogaster gouanii_, etc.; _E. linstowi_ Ham. in _Leuciscus idus_, _Abramis ballerus_, _Abramis bipunctatus_, and _Acipenser huso_; _E. lutzii_ Ham. was found by Dr. Lutz in Brazil in the intestine of _Bufo agua_; _E. angustatus_ Rud. 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 Isopod _Asellus aquaticus_. _E. moniliformis_ Brews. is stated to attain maturity in the human intestine. Except for the fact that _G. gigas_ has once been observed in the same place, this is the only human parasite amongst the Acanthocephala. Its normal second hosts are _Mus decumanus_ and _Myoxus quercinus_, and its first or larval host, the larvae of the beetle _Blaps mucronata_. _E. porrigens_ Rud. is found in considerable numbers in the small intestine of a fin-whale (_Balaenoptera sibbaldii_), and _E. strumosus_ Rud., in the small intestine of a seal (_Phoca vitulina_), and in the body-cavity of the angler fish (_Lophius piscatorius_). _E. acus_ is common in the whiting, _Gadus merlangus_.

FAMILY II. GIGANTORHYNCHIDAE.—Large forms with ringed, flattened, and _Taenia_-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.

{184}Hamann places three species in this family: _Gigantorhynchus echinodiscus_, _G. spira_, and _G. taenioides_; but as he points out that _E. gigas_ resembles these in its more important structural features, it seems advisable to include it here under the name _G. 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. gigas_ occurs in the adult state in the small intestine of swine; in Europe its first or larval host is believed to be the grubs of _Melolontha vulgaris_ and _Cetonia 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 beetle _Lachnosterna_, and he has succeeded in infecting the grub of _L. arcuata_ by feeding it on the eggs of the parasite; from one larva he took 300 parasites six weeks after feeding it. _L. arcuata_ is, like _M. 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. echinodiscus_ inhabits the intestine of ant-eaters, having been found in _Myrmecophaga jubata_ and _Cycloturus didactylus_. _G. spira_ lives in the king vulture _Sarcorhampus papa_, and _G. taenioides_ in _Dicholophus 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 clavaeceps_ and _N. agilis_, which afford interesting examples of paedogenesis. The sub-cuticle and the lemnisci are dominated by a few giant {185}nuclei, 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 of _Sialis lutaria_, one of the Neuroptera, and in the alimentary canal of the leech _Nephelis octocula_, and successful experiments have been made in infecting some species of the water snail _Limnaea_. _N. agilis_ occurs in _Mugil auratus_ and _M. 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 species _Arhynchus hemignathi_ was 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 of _Hemignathus_ consists 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.

{186}CHAPTER VII

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_, and _Krohnia_. 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, and _Spadella_ and _Krohnia_ have a single pair, and _Sagitta_ two 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.

The head is surrounded by a fold of skin, forming a hood, {187}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 as _Spadella 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 in _Spadella marioni_ it is on the upper surface.

There are no special respiratory, excretory, or circulatory organs, unless a glandular structure described by Gourret in the head of _Spadella marioni_ be a real kidney.

The nervous system consists of a supra-oesophageal ganglion {188}or 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.

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 have {189}no 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 cephaloptera_ is, 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 or _gastrula_, which opens by a pore, the _blastopore_. 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 of _Sagitta_ is 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.

HABITS.—The Chaetognatha are essentially pelagic, and {190}resemble 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 are _Sagitta hexaptera_ and _Sagitta 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 as _Calanus finmarchicus_, and small Amphipods as _Phoxus 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 another _Sagitta_ except 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 of _Obelia_, three of which are engaged in ingesting as many young _Sagitta_.

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 pass {191}through 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.) _Sagitta_ Slabber.—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. gracilis_ Verrill, _S. elegans_ Verrill, _S. darwini_ Grassi, _S. diptera_ d'Orbigny, and _S. triptera_ d'Orbigny—whose position, owing to the inadequacy of their description, is of doubtful validity.

The distribution of the other species may be mentioned. _S. hexaptera_ is the largest Chaetognath known, and reaches in the adult stage a length of 7