Chapter XII: Introduction: Nematoda—anatomy—embryology—classification—ascaridae (2)

A similar protandry exists in the parasitic forms of _Allantonema_,[189] of which there are several species—_A. mirabile_ in _Hylobius pini_, _A. sylvaticum_ in _Geotrupes sylvatica_, _A. diplogaster_ in _Tomicus typographicus_; but in their case the male and female forms which leave their host pair in the damp earth and give rise to larvae which make their way into the body of the beetle-grubs. Here they undergo very extensive retrogressive change. The body of the female, which becomes the shape of a thick sausage, is encapsuled and surrounded by a curious hypertrophied network of tracheae (Fig. 74). As is usually the case with the degenerate parasitic forms, there are practically no organs but the ovary, and this is {153}embedded in a fatty parenchyma which fills all the space within the skin.

_Atractonema gibbosum_, which lives in the body-cavity of the larva of _Cecidomyia pini_, has a similar life-history, but the parasitic form has a structural peculiarity which merits attention (Fig. 75). At the time of sexual maturity a swelling, which is caused by the prolapsus of the uterus and vagina, appears at the posterior end of the body; this swelling increases until it equals the rest of the body of the Nematode in size. Even this is far surpassed by a similar protuberance in _Sphaerularia bombi_, where the evaginated sac grows with such extreme rapidity that in a few weeks its length increases from .25 mm. to 15 mm. and its volume 60,000-fold, the increase being due, according to Leuckart, to the increase in size of the individual cells and not to their multiplication. The Nematode which has produced this enormous growth gets relatively smaller and smaller, and ultimately drops off (Fig. 76). The sexual larvae which arise from the eggs in this sac leave the body of the bee in which this species is parasitic by the anus, {154}and may live in damp earth, moss, etc., for months without taking nourishment, until the autumn, when they become sexually mature and, according to Leuckart, pair. The fertilised female is believed to bore her way into the humble-bee whilst the latter is seeking her underground winter quarters; this accounts for the fact that only queen bees are infected. The parasite is widely distributed both in Europe and North America; it is found in many species of _Bombus_, but most frequently in _B. lapidarius_ and _B. terrestris_. The presence of the _Sphaerularia_ affects the reproductive organs of the host, and reduces their fertility, so that an infected queen bee never succeeds in forming a colony.

VI. FAMILY ANGUILLULIDAE.

For the most part free living and of small size. The oesophagus has usually a double swelling or two oesophageal bulbs. The male has two equal spicula.

Genera: _Diplogaster_, _Mononchus_, _Rhabditis_, _Tylenchus_, _Anguillula_, and many others.

Many species of this family live in humus or decaying matter; others live on, or are parasitic in, plants; some, such as _Anguillula aceti_, which is found in vinegar and in paste, live in organic fluids.

The part played by the presence of these Nematodes in the soil is not thoroughly understood; sometimes they occur in great numbers, and even when not directly parasitic in plants, probably do them much damage. Cobb[190] has recently described from Australia and Fiji over eighty species, one-half of them new, which occur mostly in the earth, and many of them among plant roots. They frequently crawl up on to plants, especially on to seedlings. An instance of this is given as follows: "The edible part of three bunches of nice-looking celery bought of a Chinaman in Sydney was cut off as far up as it was tender, nearly to the first leaflets. It was washed by hand in a tin dish in tank water, free from Nematodes. The washings gave about 200 to 300 Nematodes, belonging to five different genera."

It is very probable that many of the free-living forms which have received distinct specific names may ultimately turn out to be but stages in the life-history of some of the parasitic species. {155}Von Linstow[191] has pointed out that the free form of _A. diplogaster_, if found alone, would be placed in the genus _Diplogaster_; similarly the bisexual form of _Ascaris nigrovenosa_ is known as _Rhabditis nigrovenosa_.

Those Nematodes which live parasitically in plants, _e.g._ many of the genera _Tylenchus_ and _Aphelenchus_ and _Heterodera_, as well as those which only pierce the epidermis of the roots (the remaining species of the above-named genera), are provided with a spine which works to and fro through the mouth and assists the animal to bore into the tissues of the plant. _Tylenchus devastatrix_ lives and reproduces in leaves and stems (never in the roots, except in the case of hops[192]) of many cultivated plants, such as rye, oats, onions, etc. "Clover sickness" is probably caused by this Nematode. The plants become infected by the thread-worms in the soil during the spring; their presence causes swellings and often kills the plant, in which case the worms return to the soil or remain in the straw.

_Tylenchus tritici_ Need. is the cause of "ear-cockles" in corn. These take the form of brown or purple galls, which replace the grains of corn, and which contain hundreds of minute Nematodes. In these galls they are motionless, and are capable of surviving in dryness for at least twenty years; but when moistened,—for instance, by the gall falling on damp earth,—they resume their vitality and make their way to the young wheat plants, and then, wriggling up the leaves and stems, find their way to the ear. Here they pair, and producing a gall-like growth in the flower, lay numerous eggs, from which arise the Nematodes of the ear-cockle.

_Heterodera schachtii_[193] Schmidt, is the cause of the "beet {156}sickness," and forms galls or swellings on the roots of many plants, in England especially on the roots of tomatoes and cucumbers. The free larvae live in the earth and make their way into the smaller rootlets; here the female larvae shed their skin, lose their characteristic Nematode form, and become citron-shaped (Fig. 78, D). The male larvae undergo a change, and after a period of rest cast their skin and, leaving the rootlet, seek out the females. The female does not undergo this second ecdysis, but its generative organs grow and mature in what is practically a larval stage. The embryos develop within the body of the mother, and, escaping through the uterus, ultimately cause her death. They then make their way into the earth. The cycle of the development takes but four or five weeks, so that, as in the case of _Tylenchus devastatrix_, there are several broods in a year; _T. tritici_, on the other hand, has but one.

Vuillemin and Legrain[194] point out that while _Heterodera_ is injurious to cultivated plants growing in damp soil, its presence is advantageous to those that grow in deserts. It is very common in the Sahara, and attacks many plants which are {157}immune from it elsewhere. It causes the rootlets to swell out, and the bladder-like extensions thus formed act as reservoirs for water.

Many other species attack plants; _Tylenchus millefolii_ Löw forms galls on _Achillea_, _T. dipsaci_ Kühn. on the teazle. They all seem to have great powers of resisting desiccation. The former species, when dried and placed in a herbarium in May, gave rise to active worms when moistened the following October; and the corn eel-worm is said to survive twenty-seven years in a state of suspended animation. On the other hand, although these Nematodes like moisture, they cannot withstand submersion in water for any time. They can resist a considerable degree of cold, and a species, _Aphelenchus nivalis_ Auriv.,[195] has been described from Spitzbergen, where it lives in the snow amongst a small red alga, _Sphaerella nivalis_.

VII. FAMILY ENOPLIDAE.

Small, as a rule free-living, usually marine Nematodes, without a second oesophageal bulb. Eyes and mouth-armature often present. Fine hairs and bristles sometimes surround the mouth.

Genera: _Enoplus_, _Dorylaimus_, _Enchelidium_, and others.

The genus _Enoplus_ is exclusively marine, living amongst Algae and Hydroids in shallow water and moving actively about, but never coiling into spirals. De Man[196] describes _Enoplus brevis_ Bast. as being attacked by a plant parasite, probably a Bacterium, of a greenish colour, which infested the muscles and gave them a peculiar colour.

Numerous other species have been described by De Man from the coast of Holland. It is probable that some of them are the free stages of parasitic forms; a brackish water species found in the East Indies (_Dorylaimus palustris_) is regarded by Carter as the larva of _Filaria medinensis_. _Oncholaimus echini_ Leyd. is parasitic in the intestine of the sea-urchin _Echinus esculentus_. _Tricoma cincta_[197] has a strongly striated cuticle, which gives it almost the appearance of segmentation. _Fimbria tenuis_ has numerous hairs on the tail, and the mouth is surrounded by bristle-bearing papillae.

{158}Here must be mentioned two families closely allied to the true Nematodes.

(i.) CHAETOSOMATIDAE.—This family includes three genera: _Chaetosoma_, _Rhabdogaster_, and _Tristicochaeta_. According to Metschnikoff,[198] although they are not true Nematodes, they have a great likeness to the group. He distinguishes them from the swimming members of the group as "creeping Nematoda." _Chaetosoma_, of which two species are known, _C. ophicephalum_ and _C. claparedii_, has a head distinct from the body (Fig. 79). The mouth is at the anterior end, surrounded by a double semicircle of movable spicules; the whole body is covered by fine hairs, and on the ventral surface, just in front of the anus, is a double row of about fifteen cylindrical projections, by whose agency the animal creeps. The female _C. claparedii_ is 1.5 mm. long, the male 1.14 mm. They were found creeping about on sea-weeds in the neighbourhood of Salerno.

The genus _Tristicochaeta_[199] differs from the foregoing in having three rows of locomotor projections instead of two.

_Rhabdogaster_ has no head distinct from the body, though the anterior part of the body is swollen. A second swelling occurs, as is also the case with _Chaetosoma_, in the region of the opening of the genital ducts. The female in _Rh. cygnoides_ attains a length of 0.36 mm. In this genus the hairs are confined to {159}the dorsal middle line. The locomotor projections are hooked, and are much finer than those of _Chaetosoma_, and they are situated farther forward than in the last-named genus. _Rhabdogaster_ occurs in the same surroundings as _Chaetosoma_. _Ch. ophicephalum_ is recorded from the English Channel.

(ii.) DESMOSCOLECIDAE.—The members of this family are minute, and are characterised by the presence of well-marked ridges which surround the body and give it an appearance of segmentation. The head, which is somewhat swollen, bears four bristles, and single pairs are borne by a certain number of the ridges, some on the dorsal and some on the ventral surface. These hairs can be moved independently of one another. Two red eye-spots are described between the fourth and fifth rings. The sexes are distinct, and the internal organs generally have a marked resemblance to those of the true Nematoda. The _Desmoscolecidae_ move by looping their bodies after the manner of the Geometrid caterpillars, as well as by creeping with their bristles. The genus contains numerous species[200]: _D. minutus_ Clap. (English Channel), _D. nematoides_ Greef, _D. adelphus_ Greef, _D. chaetogaster_ Greef, _D. elongatus_ Panceri, and _D. lanuginosa_ Panceri. They are exclusively marine.

_Trichoderma oxycaudatum_ Greef[201] is a minute animal, 0.3 mm. long, which has no head or ventral spines, but whose body is ringed and covered with long hair-like bristles. The male has two spicules, and the internal organisation recalls that of other Nematodes; still its ringed body has induced some authorities to place it near to _Desmoscolex_.

THE LIFE-HISTORY OF NEMATODES.

Although, considering the enormous number of species of Nematodes and the remarkable diversity of the conditions under {160}which they live, their bodily structure shows a very striking uniformity, the same is by no means the case with their life-history, which exhibits an astounding variety. Von Linstow[202] has arranged the various modifications, which occur under fourteen heads. He includes in his list the Gordian worms, which we have placed under a different heading. The following account has been taken from his paper, with a few alterations:—

1. The embryos develop, with a larval stage and without any change of medium, directly into the mature sexual forms. They live in fresh, brackish, or salt water, in plants, in the earth or in decaying organic matter: examples, _Dorylaimus_, _Enoplus_, _Plectus_, _Monhystera_.

2. The larvae live in the earth, the sexual forms in plants: examples, _Tylenchus tritici_ and _T. devastatrix_, _Heterodera schachtii_ (Figs. 77 and 78).

3. The larvae live in animals, after whose death and decay they are set free and develop into the sexual animals in the earth: example, _Rhabditis pellio_.

4. The bisexual forms live in the earth, and the fertilised females bore into animals (insects), and here produce embryos: example, _Sphaerularia bombi_ (Fig. 76).

5. The bisexual forms live in the earth; the females do not develop, but the males make their way into Insects (Beetles), and becoming hermaphrodite, develop ova which give rise to the bisexual form: example, _Bradynema rigidum_.

6. The larvae live in the earth, the sexual form in Vertebrates: examples, _Dochmius_, _Strongylus_.

7. The Nematode lives as a hermaphrodite in animals, the offspring of this, by an alternation of generations, become sexual in the earth: example, _Rhabdonema_ in Frog.

8. A bisexual free form gives origin to a bisexual parasitic form living in an animal: example, _Leptodera appendiculata_ in Snails.

9. The eggs develop in the earth, and give rise to embryos which are transferred whilst still in the egg-cell to the body of an animal. The embryos hatch out and form bisexual parasites: examples, _Oxyuris_, _Trichocephalus_.

10. The larvae live in insects, the sexual worms in water or in the earth: example, _Mermis_.

11. The larva lives encapsuled and is passively transferred to {161}a second animal: examples, _Ollulanus_, from Mouse to Cat; _Cucullanus elegans_, from _Cyclops_ to Perch; _Spiroptera obtusa_, from Meal-worm to Mouse.

12. The sexual form lives for a short time in the intestine of a Vertebrate, and produces larvae which bore through the intestinal wall and become encapsuled in the tissues: example, _Trichina spiralis_.

13. The sexual animal lives in the trachea of birds; the ova containing embryos are coughed up and are taken into other birds with food. They quit the egg-shell and wander into the air-sacs, and finally into the trachea: example, _Syngamus_.

14. There are two larval forms; the first lives in water, the second in the lungs of Amphibia, whence they wander into the intestine and become sexually mature: example, _Nematoxys longicauda_ in _Triton alpestris_.

PARASITISM.

1. EFFECT OF PARASITISM ON THE PARASITE.—The usual effect of parasitism on the parasitic organism is that the various organs necessary for a free life tend to degenerate, whilst there is a multiplication and development of organs of adhesion, by means of which the parasite maintains its hold on its host. There is further an immense increase in the powers of reproduction, which may take the form of an increase in the number of fertilised eggs produced, or the parasite may at some time of its life reproduce asexually, by budding, or fission, or parthenogetically.

Of the various classes of animals which are more or less parasitic, the Nematodes show less difference between the free-living and parasitic members of the group than obtains in any other class. With few exceptions, such as _Sphaerularia_, _Allantonema_, and one or two others, the parasitic forms have undergone but little degeneration. It is true that they have no eyes such as the free forms often possess, but in other respects, such as in the nervous, muscular, and digestive systems, they do not show any marked retrogression; further, the mouth-armature is developed in many free forms, and is not confined to the parasites.

The group has developed no methods of asexual reproduction by budding or fission, such as are found in Platyhelminthes; and the cases of an alternation of generations in which a sexual form alternates with a parthenogenetic form, are rare, _e.g._ {162}_Rhabdonema nigrovenosum_; and it seems possible that even when parthenogenesis has been described, further observation may show that the parthenogenetic stage is really a protandrous hermaphrodite, in which case the alternation of generations in Nematodes, _i.e._ the hermaphrodite alternating with the dioecious form, is a case of _heterogamy_ or the alternation of two sexual generations.

On the other hand, parasitic Nematodes produce enormous numbers of eggs. Van Beneden states that 60,000,000 have been computed in a single Nematode, and this multiplication of ova is absolutely necessary, for the chance of the embryo reaching the right host, in which alone it can develop, is always a small one.

It is a common thing to find that parasites are either hermaphrodite or that the male is degenerate, as is the case with many of the parasitic Crustacea, but with one or two exceptions the Nematoda are bisexual, and although, as a rule, the males are smaller than the females, they show no other trace of degeneracy.

In spite of the fact that the class as a whole shows but few special modifications consequent on a parasitic mode of life, it is clear that the Nematoda are peculiarly adapted for such a mode of life. Their elongated thread-like bodies afford little resistance to the passage of the food, which, as it passes through the intestine of the host, might tend to carry the parasites out of the body. At the same time their shape enables them to pierce and wriggle through the various tissues without making any very serious lesions such as might prove fatal to their host. Their extraordinary power of resisting desiccation both in the egg and in the adult state vastly increases their chances of ultimately hitting on the right host. They are capable of living in a state of suspended animation for months, and even years when dried (_vide_ p. 136), and of resuming their activity on being moistened.

The great faculty this group shows for living parasitically is evinced by the extraordinary variety of life-history presented by the different species. There is scarcely a stage which may not be parasitic; the eggs, the larvae, the adults are all in some cases free, in others parasitic, and in many cases first the one and then the other.

2. OCCURRENCE AND EFFECT OF THE PARASITE ON THE HOST.—Von Linstow states that the only law that can be derived inductively from the study of the life-history of Nematodes is that those that live in animals never pass through all their stages of development in the same organ; consequently, in considering the distribution of {163}the parasites within the body of their host we have a double habitat to consider. Many forms, such as _Trichina spiralis_, wander from the intestine to the muscles; others, such as _Filaria medinensis_, from the alimentary canal to the lymphatics or blood vessels or subcutaneous tissues. Others pass from the body-cavity to the intestine, as the Mermithidae, which infest Insects, or from the stem and leaves of a plant to its flower, as in the case of _Tylenchus tritici_.

With regard to their occurrence in the different classes of the animal kingdom, they have been most frequently observed in Vertebrates and in Insects. They are comparatively rare in the other large divisions. Many genera are confined to certain hosts: thus _Ascaris_, _Filaria_, _Trichosoma_ occur only in Vertebrates; _Spiroptera_ (with one exception) in Mammals and Birds; _Cucullanus_ in Fishes and Amphibia; _Strongylus_ and _Physaloptera_ in Mammals, Birds, and Reptiles; _Dochmius_, _Pseudalius_, _Trichocephalus_ in Mammals; _Dispharagus_, _Hystrichis_, _Syngamus_ in Birds; _Nematoxys_, _Hedruris_ in Amphibia and Reptiles; _Ichthyonema_ in Fishes; and _Isacis_ and _Mermis_ in Insects.

Twenty-two species have been described as parasitic in man, of which perhaps the most dangerous are _Filaria medinensis_, the three varieties of _F. sanguinis hominis_; _Dochmius_ (_Ancylostomum_) _duodenalis_, and _Trichina spiralis_. The Ascaridae, as _Ascaris lumbricoides_ and _Oxyuris vermicularis_, though painful, seldom cause death.

The enormous number of parasites harboured by one host is shown by the fact mentioned in Leuckart's _Parasites of Man_, that Nathusius[203] took from a single black stork 24 specimens of _Filaria labiata_ from the lungs, 16 _Syngamus trachealis_ from the trachea, more than 100 _Spiroptera alata_ from the coats of the stomach, besides several hundred Trematodes belonging to several different species (see p. 63). Even this has been surpassed in the case of a young horse, in whose body Krause found 500 _Ascaris megalocephala_, 190 _Oxyuris curvula_, several millions of _Strongylus tetracanthus_, 214 _Sclerostomum armatum_, 287 _Filaria papillosa_, 69 _Taenia perfoliata_, and 6 _Cysticercus_ forms.

It is impossible here to enter into a full description of the destruction caused to domesticated animals and crops by the presence of these parasites; full details will be found in books dealing {164}especially with this question, such as Neumann's _Parasites and Parasitic Diseases of Domesticated Animals_. A couple of cases will show how important this matter is to the farmer. Crisp estimates that _Syngamus trachealis_ causes the death of half a million pullets in England every year, and Mégnin states that in a single pheasantry 1200 victims died daily; again, the loss of one-third the crop of beetroot is by no means uncommon when it is infested with _Heterodera schachtii_. These show the practical importance of what at first sight seem quite insignificant animals, and the necessity for the minutest observation, for only when we are fully acquainted with all the details of the life-history of a parasite are we in a position to successfully combat it.

SUB-ORDER II. NEMATOMORPHA.

Until the last few years it has been customary to regard the Gordiidae as a family of Nematodes. Although in external appearance and life-history they closely resemble the members of this group, yet recent research has shown so many important morphological differences between them and the Nematoda, that most zoologists are now agreed in placing them in a different sub-Order, the Nematomorpha, a name first suggested by Vejdovsky.[204]

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The Gordiidae comprise but two genera, _Gordius_ and _Nectonema_. The latter has but one species, _N. agile_ Verr., and is marine; the former, on the other hand, is exclusively fresh-water, and contains a very large number of species. Gordian worms are frequently to be found in ditches, ponds, or large puddles, moving with an undulating motion through the water, or twining and writhing round water-plants; they are scarcer in running water. In shape they are like a piece of thin whip-cord, slightly tapering {165}at each end; the male, however, is easily distinguished from the female by its forked tail (Fig. 89). Not unfrequently a considerable number are found inextricably tangled together into a knot, and the name of the genus refers to this fact. Where numbers have suddenly appeared in water hitherto free from them, legends have sprung up which attribute their presence to a rain of worms; in reality they have come out of the bodies of Insects in which they are parasitic for the greater part of their life.

The genus _Gordius_ passes through three distinct stages, of which the first two are larval and parasitic; the third is sexually mature and lives in water. The second larval stage closely resembles the adult, but the reproductive organs are not developed. The following account of the structure of this larval form and of the adult is in the main taken from von Linstow.[206]

The whole body is covered with a well-developed two-layered cuticle, which in the adult is marked out into areas, and bears numerous minute sensory bristles, which are especially developed in the neighbourhood of the cloaca of the male. Beneath this is a hypodermis which differs markedly from the sub-cuticle of Nematodes, inasmuch as it consists of a single layer of polygonal nucleated cells. Within this lies a single layer of longitudinal muscle-cells, which differ from the corresponding layer of Nematodes in having that part of their medulla which is not surrounded by the contractile portion directed outwards towards the hypodermis, and not inwards towards the body-cavity.

The body is in the younger stages practically solid, the interior being filled with clearly defined polygonal cells which are arranged in definite rows; in later life certain splits arise in this tissue which {166}subserve various functions; between these splits strands of tissue are left which form mesenteries, and some of the cells remain lining the muscular layer (Fig. 86). These cells have been described by Vejdovsky as a definite somatic, peritoneal epithelium, but this was not found by von Linstow. Besides forming the mesenteries, and acting as packing between the various organs of the body, these cells also form the ova and the spermatozoa.

The splits which have appeared when the animal has reached the second larval stage, are two dorsal and a ventral; the latter contains the alimentary canal, and may be termed the body-cavity, the former will develop the generative organs. The mouth is occluded in the older larvae, and in the adults there is a distinct but solid oesophagus which passes into a tubular intestine. The intestine consists of a single layer of cells surrounding a lumen; it runs straight to the hinder end of the body, where it opens in both sexes with the ducts of the reproductive organs.

The nervous system consists of a well-defined circumoesophageal ring with two dorsal swellings, and, arising from this, a median ventral cord which runs the whole length of the body. The cord consists of three longitudinal strands with ganglionic cells below them; the latter, though they lie within the muscle layer, maintain a connexion with the hypodermis. Behind, the nerve-cord splits in the male, one half passing into each caudal fork. In the adult a pair of black eyes can be detected on the head; the only other sense organs are the tactile bristles mentioned above. Excretory organs are unknown.

The generative organs only attain maturity in the adult, which is, in fact, exclusively devoted to reproduction. No trace of testes {167}is found in the larva, though the two dorsal splits from the walls of which the spermatozoa will arise are present. They are lined by a definite epithelium (Fig. 83), and this serves at once to distinguish them from the body-cavity. Posteriorly the splits narrow and become the two vasa deferentia which open one on each side into the cloaca. The cells lining the lumen give rise to secondary cells, and these become spermatozoa, the process extending from behind forwards. The external organs—bursa, etc.—described by Vejdovsky were not found by von Linstow.

In the female larva two similar splits are present; these form the egg-sacs. Posteriorly they end in two short oviducts which open into a uterus, in which fertilisation takes place, and in which the secretion arises which cements the eggs together. In the adult the ovaries and a receptaculum seminis are found, in addition to the organs present in the larva. The ovaries are formed from modifications of the packing tissue; they begin close behind the head, and soon attain such dimensions as to compress the egg-sacs and body-cavity to small slits. After a time the wall between the ovary and the egg-sacs becomes absorbed, and the eggs grow into the latter. In the old females, where the egg sacs are empty, there is a considerable space round the exhausted ovary, into which eggs continue to fall off; there {168}is also a median dorsal canal which contains a few eggs. By this time the wall between the ovary and the egg-sac has again appeared.

One of the most interesting points about the female is that, according to Vejdovsky, the ovary is segmented, the cells which form the ova being heaped up in segmentally-arranged masses. This observation, if correct, is almost the only instance of segmentation recorded in the group Nemathelminthes.

The only other genus which is associated with _Gordius_ in the group Nematomorpha is _Nectonema_, of which there is as yet but one species known, _Nectonema agile_ Verr.[207] Our knowledge of the anatomy of this worm is due mainly to Bürger[208] and Ward.[209] _Nectonema_ is a marine worm found swimming near the surface of the sea with rapid undulatory motion. The males are from 50 to 200 mm. long, the females from 30 to 60 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).

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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).

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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.