The species which was discovered by Willemoes-Suhm, and named by himTetrastemma agricola, lives under stones in damp earth in the Bermudas. It differs from the other two in that the proboscis opens by a special terminal aperture. It measures nearly an inch and a half in length, and, likeG. chalicophora, is milk-white in colour. It resembles it also in possessing four eyes, and in the absence of cerebral organs and cephalic slits.
Fresh-water Forms.—In most cases the descriptions of fresh-water forms are so vague and incomplete that it is difficult to determine whether or not they are different species.
They are probably more numerous than is at present known, and are certainly scattered widely over the face of the earth, since they have been found in Nicaragua, at Tashkend in Turkestan, and at Philadelphia and Monroe in the United States.
A form of which we have a full description isTetrastemma aquarum dulcium, found by Silliman[147]at Monroe, under stones in brooks in company with Planarians. It is a small worm of a red or pink colour, about half an inch in length, and it possesses usually three pairs of eyes. The proboscis is armed, and opens by a separate aperture. The excretory system consists of a vessel on each side of the body, each opening externally by a pore, and internally dividing into numerous branches which end in ciliated expansions. An individual of the same species was found by Beddard in one of the tanks in the Botanical Gardens in Regent's Park, but as the tank is one in which tropical plants are grown, it had almost certainly been introduced among the roots of the plants, and cannot be considered as a British species.
A fresh-water Nemertine belonging to the genusTetrastemmawas, however, found by Benham[148]on the roots of some water plants in the Cherwell at Oxford. The specimen was of a bright orange colour and measured half an inch in length.
Du Plessis[149]found another fresh-water form on the lower surface of stones in shallow pools on the shores of the Lake of Geneva, and named itTetrastemma lacustre. It is a small animal, the largest specimens being rather over an inch in length.
Another European genus was found in 1893 by F. E. Schulze in Berlin. It has been fully described by T. H. Montgomery,[150]who has given it the name ofStichostemma eilhardii.
Parasitic Forms.—The genusMalacobdellawas found by von Kennel[151]in large numbers living onCyprina islandica, a Lamellibranch Mollusc, in the harbour at Kiel; and it has also been described by Riches[152]as a British form. It is attached to its host by means of a large round sucker situated at the posterior end of the ventral surface, while the rest of the body waves about freely in the mantle-cavity. It is perhaps hardly correct to describe this animal as parasitic, since it does not appear to obtain its nutriment at the expense of the host by preying on its juices. The advantage of its position is, however, obvious, since a perpetual current of water is kept up in the mantle-cavity of the Mollusc, and from the stream the Nemertine is able to pick out and take for itself any food material which it considers suitable. At the same time it is not subjected to the influence of the winds and waves, as the shell of the mollusc acts as a barrier to prevent the entrance of disturbing elements.
Malacobdellais short and broad, somewhat flattened dorso-ventrally. The anterior end is bluntly rounded. The mouth opens into a wide pharynx, which is constricted behind and then passes into the intestine; this after a few coils opens by the anus situated dorsally immediately above the sucker. The proboscis opens into the pharynx.
fig61Fig. 61.—Malacobdella grossaO. F. Müll., a large female specimen. Kiel. × 1. (From von Kennel.)A, From the dorsal surface;B, from the ventral surface.
Fig. 61.—Malacobdella grossaO. F. Müll., a large female specimen. Kiel. × 1. (From von Kennel.)A, From the dorsal surface;B, from the ventral surface.
Fig. 61.—Malacobdella grossaO. F. Müll., a large female specimen. Kiel. × 1. (From von Kennel.)A, From the dorsal surface;B, from the ventral surface.
Palaeontology.—Nemertines are unknown in a fossil state; this is probably owing to the softness of their bodies, which would render their preservation extremely improbable.
Affinities.—Until recently the Nemertines were regarded as a sub-order of the Turbellaria. They were afterwards separated from the Turbellaria and placed as a distinct class of the phylum Platyhelminthes.
Some zoologists have considered them to be so different in many respects from the other classes of the Platyhelminthes as to justify their being altogether separated from that phylum, and treated as a distinct group.
If, however, the recent work of Bürger on the excretory system is to be relied upon, the existence of flame cells would be a strong reason for classing them among the Platyhelminthes.
Hubrecht[153]has instituted an interesting comparison between Nemertines and Vertebrates. He compares the median dorsal nerve of Nemertines to the spinal cord of Vertebrates; the lateral nerve-cords to the nerve of the Vertebrate lateral line; and the lateral swellings which constitute the brain in Nemertines to the lateral ganglia of the cephalic region in Vertebrates. This view is strengthened by the existence of transverse nerves connecting the lateral and dorsal nerves of Nemertines, since these may be compared with the spinal nerves of Vertebrates. He suggests that both Nemertines and Vertebrates may have arisen from a vermiform animal possessing a nervous layer in the form of a plexus of nerve-fibres, the nerve tissue having become concentrated along three lines to form a median dorsal and two lateral nerve trunks; the former being specially developed in the Vertebrata and the latter in the Nemertines. Hubrecht further suggests that the notochord of Vertebrates may be a survival of the proboscis sheath of Nemertines, while the proboscis of the latter may be represented by the invagination to form the pituitary body in Vertebrates.
Certain authors[154]have suggested that indications exist of a relationship between Nemertines andBalanoglossus.
The features which are supposed to indicate this are the elongated vermiform shape showing no external signs of segmentation; the ciliated smooth skin and the possession of unicellular mucous glands; and the protrusible proboscis, which may be comparable to the non-retractile proboscis ofBalanoglossus, a comparison which is strengthened by the fact that in some Nemertines a sheath of nerve-fibres exists in the wall of the proboscis corresponding to the nerve plexus in the proboscis ofBalanoglossus. In both cases an ectodermic nerve plexus exists with local thickenings along definite lines, although these lines are not the same in the two cases. Both possess a straight alimentary canal, ending in a terminal anus and thrown out into paired lateral caeca, between which are the paired metamerically-arranged generative sacs.
BY
ARTHUR E. SHIPLEY, M.A.
Fellow and Tutor of Christ's College, Cambridge.
NEMATHELMINTHES
INTRODUCTION—NEMATODA—ANATOMY—EMBRYOLOGY—CLASSIFICATION—ASCARIDAE—STRONGYLIDAE—TRICHOTRACHELIDAE—FILARIIDAE—MERMITHIDAE—ANGUILLULIDAE—ENOPLIDAE—PARASITISM—NEMATOMORPHA—ANATOMY—CLASSIFICATION—LIFE-HISTORY—ACANTHOCEPHALA—ANATOMY—EMBRYOLOGY—CLASSIFICATION.
The Nemathelminthes include three sub-Orders of very different size and importance. These are—
i. The Nematoda.ii. The Nematomorpha (Gordiidae).iii. The Acanthocephala.
i. The Nematoda.
ii. The Nematomorpha (Gordiidae).
iii. The Acanthocephala.
Although the members of these groups differ considerably from one another, on the whole there is a closer resemblance between them than between any one of them and any other group of animals, and there is a certain convenience in arranging them under one head.
The following characteristics are common to all three groups of the Nemathelminthes: they are worm-like in form, and with few exceptions are parasitic in the bodies of other animals, either Vertebrate or Invertebrate. Some of them spend their whole existence within the bodies of their hosts, but more commonly they are only parasitic during a certain period of their life; a few, however, lead a free life in water or in damp earth. None of the Nemathelminthes are segmented—that is, their bodies are not divided into a number of parts which serially repeat each other, and which resemble more or less closely the preceding andsucceeding parts. They are not provided with any appendages or limbs, but sometimes bear a few bristles or hooks, and in rarer cases suckers. The body, which is elongated and, as a rule, thread-like and tapering at each end, is enclosed in a thick cuticle or hardened secretion of the underlying cells. In no Nemathelminth is there any closed vascular system, nor are special respiratory organs developed.
In many respects the most remarkable peculiarity of these animals is that, with the possible exception of the excretory organs of the Acanthocephala, there is a complete absence of cilia throughout the whole group. In this respect they resemble the Arthropoda. The universal presence of these small flickering processes of cells from man down to the simplest unicellular organisms makes the absence of these structures most remarkable. In many animals they are the sole organs of locomotion, and in almost all they perform very important functions, both in bringing food and oxygen to the body, and in removing waste matter from it. At present there seems to be no adequate explanation for their absence in the two large groups mentioned above.
Nemathelminthes are, with hardly an exception, dioecious—that is to say, their male and female reproductive organs are in different individuals. Their young do not differ markedly from the adults, except in the absence of sexual organs, but the immature stages are usually termed larvae, and not infrequently either inhabit a different host from the adult, or are free when the adults are parasitic, orvice versâ.
Sub-Order I. Nematoda.
Anatomy.—The Nematode worms, or thread-worms, form by far the largest and most important division of the group Nemathelminthes. The number of species is great, and although the conditions under which they live are of the most varied kind, there is, as a rule, little corresponding difference in structure, and hence the determination of the species is attended with no small difficulty.
With few exceptions the shape of the body is filiform (Figs. 66 and 71), the two ends being more or less pointed, and the posterior end of the male, which is generally a smaller animal than the female, is usually slightly recurved. The worms are, as a rule,white, or of the colour of polished ivory; they may be opaque or semi-transparent, but pigment spots are rarely developed.
Minute Nematodes abound in moist soil, around the roots of plants, etc., and may easily be detected with the aid of a lens wriggling about amongst the particles of sand and earth. Of the animal parasites perhaps the most familiar is the "round worm" (Ascaris lumbricoides, Figs. 66 and 67), which inhabits the alimentary canal of man; others are common in domesticated animals, asA. mystaxin the cat and dog, andA. megalocephalain the horse and ox. They are also found living parasitically in plants (Fig. 77), causing the formation of galls and other pathological growths;Anguillula(Tylenchus)triticicauses in this way considerable damage to corn, and others attack root-crops, cabbages, etc. The "vinegar eel" (Anguillula aceti), which occurs so often in weak vinegar, is another familiar example of this group.
The Skin.—The body of the worm is encased in a relatively thick, transparent, smooth cuticle, which is turned in at the various apertures, and lines the tubes connected with them for a greater or less distance. The cuticle is in some cases raised to form spikes or hooks, and in certain species,e.g.Ascaris mystaxandA. transfuga, it is produced into two lateral fins, which are supported by a thickened triradiate rod of specialised cuticle (Fig. 62); these fins, however, do not run far down the body. As a rule the cuticle is quite smooth, but it may be ringed, as inFilaria laticaudataand inF. denticulata; and the rings may bear backwardly-projecting teeth.
The skin of Nematodes consists of three layers—(i.) the above-mentionedcuticle, which is presumably secreted by (ii.) thesub-cuticleorepidermiswhich underlies it; the latter surrounds in its turn (iii.) themuscular layer.
The nature of the sub-cuticle is one of the debateable points in the morphology of the Nematoda. No cell outlines have been detected in it, although nuclei are scattered through it; it is in fact asyncytium, or protoplasmic mass in which cell limits cannot be distinguished. Many of the cells forming it have broken down into fibrils, and these form a close meshwork, which is occasionally specialised, as, for instance, round the nerve-cords. Along the median dorsal and ventral lines, and along the lateral lines, this tissue is heaped up in such a way as to divide theenclosed muscle-cells into four quadrants. These thickenings surround dorsally and ventrally a specialised nerve-cord, and laterally the excretory canals.
According to Jammes[155]this lack of differentiation in the sub-cuticular layer is caused by the early appearance of the cuticle, which he thinks is necessitated, at any rate in many of the parasitic forms, by the action which the digestive juices of the host would have on the otherwise unprotected body-wall.
fig62Fig. 62.—A transverse section through the body ofAscaris transfugaRud., in the region of the oesophagus:a, the muscular oesophagus with its triradiate lumen;b, the cuticle;c, the sub-cuticle;d, the muscular layer;e, the lateral nerves running in the lateral line;f, the excretory canal;g, the dorsal, andh, the ventral nerve;i, the triradiate rod in the fin.
Fig. 62.—A transverse section through the body ofAscaris transfugaRud., in the region of the oesophagus:a, the muscular oesophagus with its triradiate lumen;b, the cuticle;c, the sub-cuticle;d, the muscular layer;e, the lateral nerves running in the lateral line;f, the excretory canal;g, the dorsal, andh, the ventral nerve;i, the triradiate rod in the fin.
Fig. 62.—A transverse section through the body ofAscaris transfugaRud., in the region of the oesophagus:a, the muscular oesophagus with its triradiate lumen;b, the cuticle;c, the sub-cuticle;d, the muscular layer;e, the lateral nerves running in the lateral line;f, the excretory canal;g, the dorsal, andh, the ventral nerve;i, the triradiate rod in the fin.
The nervous system, according to the same writer, is of the same nature as this sub-cuticular tissue, only it is more differentiated, or perhaps we should say it has retained more of the primitive cellular character of the embryonic tissue. The fibres of the sub-cuticular tissue are closely connected with the fibrils which compose the spongioplasm (Fig. 64,d) of the muscles,[156]and form also the sheaths of the various nerves; in fact the passage of these fibrils into the nerves is so gradual that it is impossible to make any separation between them.
The Nervous System.—The central organ of the nervous system is the circumoesophageal ring which surrounds the pharynx, close to the anterior end of the body, inA. megalocephala1½ to 2 mm. behind the mouth.[157]Ganglion cells are found in the ring, but they are not numerous, and are chiefly aggregated round the points of origin of the nerves.
Six short nerves, three on each side of the median line, run forward from the ring, a pair of these ending in each of the three papillae which surround the mouth.
Behind, the nerve-ring gives off six main nerve trunks, of which the dorsal and ventral nerves are usually the largest. These run in the median dorsal and ventral thickenings of the sub-cuticular tissue, and are connected one with another by numerous fine lateral branches running through the sub-cuticle.
fig63Fig. 63.—Diagram of the nervous system at the two ends of the body inAscaris megalocephalaCloq., ♂. (After Hesse.)a, Circumoesophageal nerve-ring;b, opening of excretory ducts;c, dorsal nerve;d, dorso-lateral nerve;e, ventro-lateral nerve becoming the bursal nerve posteriorly;f, the ventral nerve;g, cloacal opening;h, sub-cuticular nerves running fromctof;k, spicules.
Fig. 63.—Diagram of the nervous system at the two ends of the body inAscaris megalocephalaCloq., ♂. (After Hesse.)a, Circumoesophageal nerve-ring;b, opening of excretory ducts;c, dorsal nerve;d, dorso-lateral nerve;e, ventro-lateral nerve becoming the bursal nerve posteriorly;f, the ventral nerve;g, cloacal opening;h, sub-cuticular nerves running fromctof;k, spicules.
Fig. 63.—Diagram of the nervous system at the two ends of the body inAscaris megalocephalaCloq., ♂. (After Hesse.)a, Circumoesophageal nerve-ring;b, opening of excretory ducts;c, dorsal nerve;d, dorso-lateral nerve;e, ventro-lateral nerve becoming the bursal nerve posteriorly;f, the ventral nerve;g, cloacal opening;h, sub-cuticular nerves running fromctof;k, spicules.
The lateral nerves, which consist of two or four bundles, one or two lying dorsal and one or two ventral to each excretory canal, have a double origin. The dorsal branches arise directly from the nerve-ring, and at their point of origin there is a considerable accumulation of ganglion cells, from which two commissures on each side run into the ventral nerve (Fig. 63,f). The ventral branches arise from the ventral nerve-cord immediately in front of the excretory pore. At the posterior end the lateral nerves pass into the two branches into which the ventral nerve divides. Just before the point where the ventralnerve splits it swells out into an anal ganglion situated just in front of the anus. In the male[158]this anal ganglion gives off two lateral nerves which pass round the cloaca and form a ring, and in this sex the ventro-lateral nerve, which is much strengthened by fibres from the ventral nerve, and has received, owing to the mistaken impression that it was a specialnervus recurrens, the name of the "bursal nerve," gives off numerous branches to the sense papillae which are found in this region of the body and on the tail. The arrangement of these parts is shown in Fig. 63.
Sense organs are but poorly developed in the Nematoda, as is usual in animals which are, as a rule, either parasitic or live underground. Eyes, consisting of masses of dark pigment with or without a lens, occur in the neighbourhood of the circumoesophageal nerve-ring in some free-living forms. Leuckart described as possible auditory organs certain giant-cells lying near the orifice of the excretory ducts. Later research has shown these cells to have some phagocytic action on the contents of the body-cavity. The chief sense organs are the papillae, of which inA. megalocephalathere are two kinds, the lip papillae being distinguished from the genital papillae by the fact that the nerve supplying them ends in a fine point and pierces the cuticle in the former case, whilst in the latter it swells out into an "end-organ," which is always covered by a layer of cuticle, though sometimes by a very thin one.
Muscular System.—The muscular system is one of the most characteristic features of the Nematoda, both as regards the histology of the muscle-cells and the way in which the cells are arranged.
Each muscle-cell is of considerable size, and is of the shape of a somewhat flattened spindle produced into a process near the middle. Each end of the spindle cell is said to be continuous with the fibrils of the sub-cuticular layer.[159]The muscle-cell consists of two portions, a contractile part which lies next the sub-cuticle, and which usually, to some extent, wraps round the second or medullary half. The latter consists of a fibrillar spongioplasm, in the meshes of which lies a clear structureless hyaloplasm. The nucleus always lies in the medullary half.
The contractile portion consists of a number of columns, very regularly arranged in two rows and close together, but allowing sufficient space between adjacent columns for fibrils of the spongioplasm to penetrate; and these become continuous with the fibrils of the sub-cuticle, which is thus intimately connected with both nervous and muscular systems.
The medullary portion of the cell varies greatly in size; it may stretch far into the body-cavity, which may be thereby almost occluded, or it may be flattened out, leaving a large space around the alimentary canal. At one point, usually about its middle, it is produced into a process, which bends inwards towards the dorsal or ventral nerve-cord, and by means of this process the muscle receives its nerve supply.
In most Nematodes there are numerous muscle-cells to be seen in any transverse section, forming a layer within the sub-cuticle, and broken up into four quadrants (Fig. 62) by the projection of the dorsal, ventral, and lateral thickenings of the sub-cuticular tissue. In some genera, however, such asOxyuris,Strongylus,Pelodera,Leptodera, etc., there are but eight muscle-cells in a row, two in each quadrant. Such genera are classed together by Schneider,[160]and termed Meromyarii (videp.137).
fig64Fig. 64.—A, transverse section through the centre of a muscle-cell;B, the same through a nerve fibre showing the sub-cuticular fibres running into the sheath. (After Rohde.)a, Cuticle;b, sub-cuticular fibres continuous withd;c, contractile columns;d, network of spongioplasm;e, nucleus.
Fig. 64.—A, transverse section through the centre of a muscle-cell;B, the same through a nerve fibre showing the sub-cuticular fibres running into the sheath. (After Rohde.)a, Cuticle;b, sub-cuticular fibres continuous withd;c, contractile columns;d, network of spongioplasm;e, nucleus.
Fig. 64.—A, transverse section through the centre of a muscle-cell;B, the same through a nerve fibre showing the sub-cuticular fibres running into the sheath. (After Rohde.)a, Cuticle;b, sub-cuticular fibres continuous withd;c, contractile columns;d, network of spongioplasm;e, nucleus.
In addition to the characteristic muscles of the body-wall there are others, such as those which move the spicules in the male, which cross the body-cavity obliquely near the anus, and such as sphincter muscles near the latter orifice, which have notthe characteristic arrangement of contractile and medullary parts described above.
The Body-Cavity.—The skin of a Nematode, as described above, contains most of the important organs of the body within its thickness. The chief muscular system, the nervous system with its sense organs, and the excretory organs are all embedded in or form part of the skin, which in its turn encloses a cavity—the body-cavity—in which the other two systems of organs which are found in Nematodes lie. These are the digestive system and the reproductive system.
The body-cavity is continuous from one end of the animal to the other, and is in no case divided up into compartments by the presence of septa or mesenteries. It contains a coagulable fluid with numerous corpuscles; this is, as a rule, colourless, but inSyngamus trachealisSieb. (Fig. 70), which lives on blood, the haemoglobin of its host tinges it red, though the colour is said to disappear if the parasite be isolated and starved.
The morphological nature of this body-cavity affords an interesting problem. It is not a truecoelom, such as exists in the earthworm, since it is not surrounded by mesoderm, nor do the excretory organs, with the possible exception of one or two genera, open into it, nor do the generative cells arise from its walls. Essentially it is a space between the mesodermic muscle-cells which line the skin and the endodermic cells of the alimentary canal, and although in many of its functions it resembles the coelom of other animals, its morphological character is quite different.
There are no respiratory or circulatory organs in the Nematoda; possibly the fluid in the body-cavity acts, to some extent, as a carrier of oxygen, but from the inert and almost vegetative life of these animals it seems probable that their respiratory processes are slow, and in fact Bunge[161]has shown thatAscaris mystax, found in the intestine of the cat, will live for four or five days in media quite free from oxygen, and thatA. acusfrom the pike will live and exhibit movements in the same media for from four to six days.
The Digestive System.—The mouth of the Nematoda is usually anterior and terminal, and is surrounded by from two to six projecting lips, the most common number being three. Theselips are well provided with sense papillae. The mouth leads into an alimentary canal, which with hardly an exception runs straight through the body to the anus without twists or loops. The anus is usually placed ventrally and is not terminal, but inTrichinaandTrichocephalusit is at the end of the body, and inMermis, where the several parts of the alimentary canal are said not to communicate, it is absent altogether.Ichthyonema,Dracunculus,Allantonema,Atractonema, and other Filariae are also aproctous.
The alimentary canal is divisible into three parts—(i.) the oesophagus, (ii.) the intestine, and (iii.) the rectum. The suctorial oesophagus is a very muscular, thick-walled tube, lined with cuticle continuous with that which covers the body, and like it cast from time to time. Its lumen is usually much reduced, and is almost invariably triangular or triradiate in section (Fig. 62). In many genera the hinder end of the oesophagus is swollen into a muscular bulb, which is armed with teeth inHeterakis,Oxyuris,Pelodera,Leptodera, etc. Other species, such asTylenchus,Aphelenchus,Dorylaimus, are armed with a spear, which inOnyx,[162]a genus recently described and allied to the last named, is borne on a special bulb. The use of the spear is to pierce the tissue upon the juices of which the animal lives. A gland lies embedded in the thick walls of the oesophagus, and opens into its lumen by a fine tube. This was first described by Schneider[163]inA. megalocephala, and more recently it has been found by Hamann[164]in a number of Ascaridae and Strongylidae from the Adriatic, and also inLecanocephalus.
With a few exceptions, such asMermis, where it is blind, the oesophagus opens posteriorly into the intestine. This is a somewhat flattened tube, whose shape and position are often altered by the development of the generative organs. Its wall consists of a single layer of columnar cells, with large nuclei coated internally and externally by a layer of cuticle. The inner layer of cuticle is usually perforated by very numerous minute pores. In some species the intestine is degenerate, inMermisit is a closed tube opening neither into the oesophagus nor into the rectum; inTrichina spiralisand in the larva ofTylenchus triticiit consists of a single row of cells perforated by a duct, but in the adult of the last named there are many cells in a transverse section.
fig65Fig. 65.—A longitudinal section through the body ofStrongylus filariaRud. (From O. Augstein.[165]) A portion of the body, on each side of the excretory pore, is seen in optical section.a, Mouth;b, oesophagus;c, intestine;d, excretory canal;e, excretory pore, and the opening of the poison glands,i;f, circumoesophageal nerve-ring;g, ventral nerve;h, dorsal nerve;i, unicellular poison glands;k, ovary, with the ova separate;l, oviduct;m, uterus, the first egg in the uterus is surrounded by spermatozoa;n, opening of uterus;o, inner end of ovary with the ova undifferentiated.
Fig. 65.—A longitudinal section through the body ofStrongylus filariaRud. (From O. Augstein.[165]) A portion of the body, on each side of the excretory pore, is seen in optical section.a, Mouth;b, oesophagus;c, intestine;d, excretory canal;e, excretory pore, and the opening of the poison glands,i;f, circumoesophageal nerve-ring;g, ventral nerve;h, dorsal nerve;i, unicellular poison glands;k, ovary, with the ova separate;l, oviduct;m, uterus, the first egg in the uterus is surrounded by spermatozoa;n, opening of uterus;o, inner end of ovary with the ova undifferentiated.
Fig. 65.—A longitudinal section through the body ofStrongylus filariaRud. (From O. Augstein.[165]) A portion of the body, on each side of the excretory pore, is seen in optical section.a, Mouth;b, oesophagus;c, intestine;d, excretory canal;e, excretory pore, and the opening of the poison glands,i;f, circumoesophageal nerve-ring;g, ventral nerve;h, dorsal nerve;i, unicellular poison glands;k, ovary, with the ova separate;l, oviduct;m, uterus, the first egg in the uterus is surrounded by spermatozoa;n, opening of uterus;o, inner end of ovary with the ova undifferentiated.
In some genera,LeptoderaandPelodera, the lumen of the intestine at any one level is bounded by two horseshoe-shaped cells, but by far the commonest arrangement is a tube formed of fairly numerous columnar cells crowded with granules and with large nuclei.
The rectum is usually short; its cuticular lining, like that of the oesophagus, is cast at intervals. At its anterior end there is usually a sphincter muscle, and its walls are divaricated by muscular strands which run from it to the body-wall. The anus is a transverse slit, which in the male Strongylidae is surrounded by a funnel-shaped membrane.
The food of Nematodes seems to be almost entirely fluid, and consists, at any rate in the parasitic forms, of the elaborated juices of their hosts. Little is known about the nutriment of the free-living forms.
The Excretory System.—The excretory organs are peculiar, and, like many other Nematode structures, do not fall readily into line with what is known of similar organs in other animals. They consist of two canals embedded in the lateral thickenings of the sub-cuticular tissue. The canals end blindly behind, but near the anterior end of the body they bend inwards, and after uniting, open by a common pore situated in the middle ventral line, a little way behind the mouth. The lateral canals are in some cases continued in front of the transverse branch, and they then end blindly in the head. The walls of these canals consist of an internal, structureless, refractive layer surrounded by a granular layer with nuclei. They contain a fluid, but nothing is known of its composition.
An interesting divergence from the usual form of excretory organ has been described by Hamann[166]in the genusLecanocephalus. Here there is only one canal, the right; anteriorly this bends towards the ventral surface and opens by a small median pore close behind the nerve-ring. Posteriorly the canal does not extend much beyond the middle of the body, where it forms a coiled mass, and diminishing in size, opens into the body-cavity. The same author also states that both canals inDochmiushave a similar internal opening; these observations, if confirmed,[167]show a conformity to the ordinary structure ofexcretory organs which was not supposed to exist in the lateral canals of the Nematoda.
The Reproductive Organs.—With the exception of the generaAngiostomum,Pelodytes, and ofRhabdonema nigrovenosum, which are physiologically hermaphrodite and self-impregnating, the Nematodes have separate sexes. The males are, as a rule, smaller than the females, and may usually be distinguished by the posterior end of the body being curved towards the ventral surface; a genital bursa, and one or more spicules are often found in this sex. Further, the position of the genital opening differs; in the male the vas deferens opens on the ventral surface of the rectum close to the anus, but the oviduct in the female opens in the ventral middle line, usually near the middle of the body, but sometimes close behind the excretory pore, or in some Strongylidae just in front of the anus. The tail of the male bears very numerous papillae, which are of considerable systematic importance.
fig66Fig. 66.—Ascaris lumbricoidesCloq. ♂, natural size, cut open along the dorsal middle line.a, Oesophagus;b, intestine;c, testis;d, vas deferens;h, lateral excretory canals.
Fig. 66.—Ascaris lumbricoidesCloq. ♂, natural size, cut open along the dorsal middle line.a, Oesophagus;b, intestine;c, testis;d, vas deferens;h, lateral excretory canals.
Fig. 66.—Ascaris lumbricoidesCloq. ♂, natural size, cut open along the dorsal middle line.a, Oesophagus;b, intestine;c, testis;d, vas deferens;h, lateral excretory canals.
With rare exceptions,e.g.Filaria attenuata, where it is double, the male reproductive organ consists of a single tube divisible into a testis proper, a vas deferens, a vesicula seminalis, where the spermatozoa are stored up, and a ductus ejaculatorius. The tube stretches through the body in a straight line in the small free-living forms, but is thrown into loops and coils in the larger parasitic Nematodes. Within the testis the mother-cells of the spermatozoa are attached to a rhachis or axial cord; the mother-cells divide, and their products ultimately form spermatozoa. The latter have a very peculiar shape; in accordance with the universal absence of cilia in the Nematoda the spermatozoon has no flagellum, and at first consists of a spherical nucleated cell, on one side of which a cap or covering of some refractive substance appears. The cap elongates andbecomes conical, whilst the protoplasmic portion of the spermatozoon throws out pseudopodia and becomes amoeboid, but ultimately rounds itself off again. The spermatozoa do not attain maturity until they reach the uterus of the female.
The internal female reproductive organs are, with few exceptions (Trichina, etc.), double, but the vagina, which is lined with cuticle continuous with that covering the body, is always single. They are usually much coiled, and may be divided into ovary, oviduct, and uterus. The ova arise from a polynucleated mass of protoplasm or syncytium (Fig. 65,o) at the upper end, and acquire distinctness as they approach the oviduct. Fertilisation takes place in the uterus, but the segmentation may not begin until some time after the eggs are laid; inDochmius, however, it is well advanced at this period, and in many genera,e.g.Pseudalius,Trichina,Dracunculus, etc., the whole development of the larva takes place in the body of the mother.
fig67Fig. 67.—Ascaris lumbricoidesCloq. ♀, natural size, cut open along the median dorsal line to show the internal organs.a, The muscular oesophagus;b, the intestine;c, the ovary;d, the uterus;e, the vagina;f, the external opening;h, the excretory canals;i, their opening.
Fig. 67.—Ascaris lumbricoidesCloq. ♀, natural size, cut open along the median dorsal line to show the internal organs.a, The muscular oesophagus;b, the intestine;c, the ovary;d, the uterus;e, the vagina;f, the external opening;h, the excretory canals;i, their opening.
Fig. 67.—Ascaris lumbricoidesCloq. ♀, natural size, cut open along the median dorsal line to show the internal organs.a, The muscular oesophagus;b, the intestine;c, the ovary;d, the uterus;e, the vagina;f, the external opening;h, the excretory canals;i, their opening.
Embryology.—The eggs of many of the parasitic forms require a considerable degree of warmth to develop. Those ofAscaris lumbricoidesrequire a temperature of 20° C., those ofTrichocephalus22.5° C., and those ofOxyuris vermicularis, 40° C. The latter develop in a few hours, the eggs ofDochmiusin a few days, whilst those ofA.lumbricoidestake weeks or even months, and the young ofTrichocephalusseldom develop within a year.[168]The ova only develop in a damp atmosphere, and they can be arrested at almost any stage, and for considerable periods, by desiccation.
Our knowledge of the processes by which the fertilised egg-cell develops into the larva is very imperfect. As a rule the segmentation is complete and equal; it results in the formation of a blastula, which may take the form of a hollow sphere of cells—A. megalocephala—or the cavity may be reduced, and the blastula may consist of a double-layered plate, as inCucullanus.[169]The distinction into cells which will form the three embryonic layers, the ectoderm, mesoderm, and endoderm, is very early evident,—in the eight-cell stage. By the growth of one side of the blastula and the tucking in of the other the blastula becomes converted into a gastrula, which is a two-layered stage with a cavity opening to the exterior by a pore termed the blastopore. In Nematodes the blastopore is elongated and slit-like; it either forms the mouth (Cucullanus) or closes from behind forwards, the mouth ultimately arising at the point where the blastopore finally closed (Rhabdonema nigrovenosum). The mesodermal cells lie between the ectoderm and the endoderm; they ultimately develop into the muscles of the body-wall, the lateral excretory canals, and the reproductive organs; the last-named two systems arise each[170]from a single cell. The nervous system arises from the ectoderm, which also forms the sub-cuticle, and is turned in slightly at the mouth and anus; the remainder of the alimentary canal develops from the endoderm.
The post-embryonic development, which is very variable, and in many cases very extraordinary, will be dealt with under the several families.
Classification.—The classification of the Nematodes is a matter of very considerable difficulty; their structure is unusually monotonous, and, owing perhaps to their largely parasitic mode of life, they show practically none of those external features which are so useful to the systematist in other groups. Schneider in his Monograph divides the group into three subdivisions—(i.)thePolymyarii, in which numerous muscle cells are seen in a transverse section; (ii.) theMeromyarii, in which only eight are seen, two in each quadrant; and (iii.) theHolomyarii, in which the muscles are either not divided, or only divided by longitudinal lines. This grouping has, however, to some extent broken down, since Bütschli[171]and others have shown that the third subdivision is founded on insufficient observation, whilst the first two include, in different subdivisions, Nematodes which are closely allied in all respects except as regards their muscle cells.
The details of the life-history have been used by other writers as a basis of classification. Linstow[172]enumerates fourteen distinct modifications of the post-embryonic development (videp.159), and Örley[173]has grouped these under three headings. The animals which fall under each group to some extent resemble one another in structure. Örley's groups are:—
(i.)Nematozoa.—Thread-worms with free larval life, the mature forms being parasitic in animals. Enormous numbers of eggs are produced, and the development is indirect. The genital organs are complicated by many convolutions.
(ii.)Rhabditiformae.—Small, as a rule microscopic, thread-worms, usually living free, but rarely parasitic. They become sexually mature only in decomposing organic substances, or in earth saturated with such substances. They live gregariously and do not produce immense numbers of ova. The metamorphosis is slight, or is complicated by sexual metamorphosis. The oesophagus has two dilatations. The genital tubes are simple and not coiled.
(iii.)Anguillulidae.—Small microscopic thread-worms, with a free existence in mould or water, throughout all stages. They produce large eggs. They are provided with a caudal sucker and bristles, sometimes with eyes and other structures characteristic of a free life. Genital tube simple and not coiled.
The disadvantage of such a system is, that to accurately place a specimen in its proper class we must be acquainted with its life-history, and this is known in but few cases.
The determination of the species to which a Nematode belongs is a matter of considerable difficulty. Amongst the more important features for purposes of classification are the arrangement of themuscles, the character of the tail in the male, especially when papillae are present, the number and the size of the spicules, and the arrangement of the lips and mouth-parts generally.
Cobb[174]has recently devised an ingenious formula in which measurements of different parts of the body appear as percentages of the whole length of the body. The nature of this will be understood by reference to Fig. 68. Such a formula should, however, be used with caution, since it rests on the assumption that the proportions of the various parts of the body are constant in different individuals, and it is by no means certain that this is the case.