Anatomy of the Nematodes

All the Nematodes are covered by (1) a CUTICLE, which in the small species is thin and delicate, while in the larger species it is thickened, and may consist of several layers of complicated structure. Canalicular pores do not occur. According to general opinion, which is confirmed by the history of development, the cuticle is a product of (2) the EPITHELIUM or ectoderm that had formerly existed or is still found beneath it; in young specimens and small species it is perceptible, but in older worms it frequently alters so considerably that not only do the borders of the cells disappear, but a fine fibrous differentiation appears in their cytoplasm. The matrix or ectoderm then has the appearance of an ectodermal syncytium permeated by fibres and strewn with nuclei, so that it is hardly distinguishable from the tissue of (3) the CUTIS, which is always present, though developed to a varying degree. Both layers, matrix and cutis, project internally as ridges and form the lateral lines, while the less marked median lines are produced apparently only by the ectoderm (fig. 260).

Unicellular cutaneous glands are known in parasitic as well as in free-living species; they vary in number and arrangement, and are found discharging some at the anterior extremity and others in the vicinity of the genital orifices. In other cases large numbers of them are present along the lateral lines; they are strongly developed in most of the Trichotrachelidæ, where they discharge either along a part of the ventral surface or along the lateral and median lines; they are placed so closely together that the ridges of the cuticle perforated by the orifices have long been known, and have been described, as “rodlet borders,” or “fields of rods.”

As the cutis is immediately adjacent to (4) the DERMO-MUSCULAR TUBE the simple layer of the muscular cells is divided into four quadrants by the longitudinal lines—two dorsal and two ventral (fig. 260). The MUSCLES are in the simplest cases large rhomboid cells that lie two by two in each quadrant, so that on transverse section of the entire worm only eight cells are perceptible. The outer border of the cells is converted into contractile fibrils, while the contiguous inner portion has remained protoplasmic, and contains the nucleus. In large species the muscular cells do not only increase in length (up to 3 mm.) and in number in every quadrant, but their contractile portion curves up to form a groove (like that of a dead leaf) thereby even becoming thicker; simultaneously space is gained for more cells, the protoplasmic parts of these cells (on transverse section) project out of the grooves like vesicles. In all cases there is only one layer of longitudinal muscular cells, which, by contracting, can shorten the body or, by contracting one side, can bend it. In the latter case the muscles of the opposite side have an antagonistic effect, or when all the muscles are contracted, the elasticity of the cuticle acts in the same way. Special muscles exist at the beginning of the gut and at sections of the genital apparatus.

The existence of a cavity between the body and the gut wall has hitherto been generally assumed, and has been referred to the cleavage cavity, and consequently designated as a primary body cavity. More recent investigators, however, state that such a cavity does not exist, but that the space between the longitudinal muscles or their protoplasmic portions and the gut epithelium is filled by a complicated “isolation tissue.” This in the main proceeds from a large cell (Is., fig. 262) which lies directly behind the nerve ring dorsal to the œsophagus, and consists of a system of lamellæ which sheathe the muscles and penetrate through them to the cutis and also cover the gut in a thin layer.

We may now consider the “tuft-like” or “phagocytic” organs, which attain 1 cm. in size, and consist of four, six, or even more ramified cells, which lie close to the walls of the body (fig. 261). They are found either only in the anterior part of the body (Ascaris), or throughout the whole length of the body (Strongylus, syn., Sclerostomum), and their position usually corresponds to the lateral lines. In some species there are small protoplasmic cells on the processes of these organs. In consequence of their size they can be recognized with the naked eye, especially when they are loaded with granules of stain (carmine, Indian ink) injected into the body cavity.

Intestinal canal.—The oral aperture, which is situated at the tip of the anterior extremity, is frequently surrounded by thick lips, or small bristles, or papillæ; it leads to a more or less strongly developed buccal cavity, which is lined by a continuation of the body cuticle, and which in some species is provided with “teeth,” representing differentiated portions of the cuticle.

The œsophagus (fig. 262), which arises from the base of the oral cavity, is as a rule a short, bottle-shaped tube with triradiate lumen; its wall is chiefly composed of radiating muscular fibres, which give it the appearance of being transversely striped when viewed from the surface. There exist also in its wall three large gland cells (œsophageal glands) and nerves arising from the lateral lines and running forward. The radial fibres cause a dilatation of the lumen, and exercise an effect antagonistic to the elasticity of the cuticle lining the inner surface. The latter has its own particular layer, which is not in direct connection with that of the oral cavity. Special dilator muscles, arising from the dermo-muscular tube and situated at the commencement of the œsophagus, are only known in a few species. The posterior end of the œsophagus presents a bulb-like dilatation, and is frequently provided with small chitinous movable valves. In a few forms, which belong to the Trichotrachelidæ (Trichocephalus, Trichinella), the œsophagus is a very long cuticular tube, beset on its dorsal surface with a series of large nucleated cells. In others (Cucullanus, Ascaris, etc.), a tube, the so-called glandular stomach, lined only by epithelial cells, follows behind the muscular œsophagus. This glandular stomach is, from its structure, easily distinguished from the mid-gut, or chyle intestine, which is likewise cellular. The so-called mid-gut is a tube lined by flat, cubical, or cylindrical cells (fig. ) surrounded by “isolation tissue”; its transverse section is circular or flattened dorso-ventrally; the lumen may run in a straight line, or it runs a sinuous course through the alternating prominences of the then flat epithelial cells.

The ectodermal hind gut is, as a rule, very short. At the anal opening the cuticle and the subcuticular layers are reflected inwards, forming the lining of the hind gut. In large species the subcuticular tissue forms large cells on which anteriorly lie in addition large “glandular cells.” In the male the ejaculatory duct opens at this point. Around the end part of the gut, either on the chyle intestine or at the beginning of the end gut, there exists a sphincter muscle arising from a muscle cell which acts antagonistically to the two diaphragm-like dilator muscle cells which stretch from the gut to the body wall. In many species large stretches of the gut are provided with dilator muscles. There is sometimes a retrogressive absorption of the gut in the adult stage of a few parasitic species.

Intestinal cæca and œsophageal glands sometimes exist as intestinal appendages; the former are tubular appendages of various size, running backwards or forwards, and arising from the posterior extremity of the œsophagus. They are lacking in many species. The œsophageal glands are unicellular; a dorsal and two subventral glands may be distinguished according to their position; as a rule they open into the œsophagus at a distance from one another. The body of the gland lies in the bulb of the œsophagus, or in the dorsal cul-de-sac arising from it.

The nervous system is sufficiently known in a few species only; it consists of a ring containing fifty to sixty fibres closely surrounding the œsophagus, various groups of ganglion cells, and a certain number of nerves extending anteriorly as well as posteriorly. The remarkably small number of fibres, as well as ganglion cells, is characteristic of the nervous system of all Nematodes. Immediately behind the œsophageal ring (fig. 263, Lg.) an agglomeration of ganglion cells lies at either side (lateral ganglia); part of their off-shoots form the œsophageal ring, and part are directed posteriorly and ventrally, and unite partly in front of and partly at the back of the excretory pore, with fibres originating direct from the œsophageal ring, and passing along the ventral median line to the back; these fibres then together form the ventral median nerve (fig. 263, V.m.n.). This nerve, originally consisting of thirty to fifty fibres, becomes in the female attenuated quite evenly in its further course. There is also an agglomeration of ganglion cells close in front of the anus (anal ganglia), and then the median nerve divides in order to combine with the lateral nerves on either side. In the male the median nerve enlarges to nearly the original number of fibres in front of the anal ganglion, which contains seven cells; there is also an anal ring embracing the terminal gut, and there are two ganglion cells in it on each side. In the dorsal median line the dorsal median nerve is alike in both sexes; arising in front with a single root from the œsophageal ring, it gathers its fibres from the lateral ganglia; in the anterior part of the body it consists of thirteen to twenty fibres; in the posterior part of the body the fibres are reduced to four or six; behind the anus it divides and combines with the lateral nerves; the latter consists of two fascicles at either side right up to their most posterior extent—one dorsal and one ventral—which in the greater part of the body do not run in, but beside the lateral lines, and exhibit a different origin anteriorly. The ventral fascicle at each side branches off from the ventral median nerve in front of the excretory pore, whereas the dorsal fascicles originate from the œsophageal ring close to the lateral ganglia. Each of the four fascicles contains only two or three fibres, which run backwards parallel to the lateral lines; a few centimetres in front of the caudal extremity they enter the lateral lines and remain separate from one another up to the level of the anal ganglion; here they amalgamate on either side, after each interpolating one ganglion cell, with the single short lateral nerve which first takes up the forked ends of the ventral and then of the dorsal median nerve; finally, both lateral nerves unite with each other at the back in an arch-like manner.

In the male each ventral part of the lateral nerves becomes thickened by taking up fibres from the ventral nerves, which become thickened posteriorly to the nervus bursalis, which towards the middle gives off a mass of fibres to the “genital papillæ” situated in front of and behind the anus; the number of these fibres averages eighty to 100; in its further course the bursal nerve resembles the corresponding ventral part of the lateral nerves of the female.

The ventral and dorsal nerves are connected by a number of semicircular commissures, which originate from the ventral nerves and serve to supply the dorsal nerve, which is always being decreased by fibres departing from it. It is remarkable that these commissures are not placed symmetrically, and their position also is different in the two sexes; in the female there are thirty-one on the right side and only thirteen on the left side. In the male there are thirty-three commissures on the right side and fourteen on the left, which run into the subcuticular layer, generally in pairs, and usually cross at the level of the lateral lines.

The fibres of the two median nerves are chiefly motor; fascicular processes run from each protoplasmic part of the muscular cells to the median nerves; from these they take up bundles of primitive fibrils, which separate, pass through the protoplasmic part and enter the contractile part (fig. ). One part of the fibrils, however, penetrates beyond the muscles into the subcuticular layer, where they form a network, probably of a sensory nature, with contiguous fibrils. Nerves directed anteriorly finally originate from the œsophageal ring; they consist each of three fibres, carry three ganglion cells at their point of origin, and enter the sensory organs of the three papillæ surrounding the oral aperture. Two of these little trunks lie in the lateral lines, the remaining four are situated in the middle of the four quadrants (Nn. sub-mediani anteriores).

Parasitic species lack higher ORGANS OF SENSE; free-living worms occasionally have two rust-red eyes, sometimes with lenses, at the anterior part of the body. In addition to the above-mentioned sensory papillæ surrounding the oral aperture and the genital papillæ of the male at the end of the body, another pair exist in the vicinity of the lateral ganglia, the “cervical papillæ,” and two dorsal papillæ in the central region of the body and two lateral ones near the tip of the tail (Ascaridæ). The differences in the distribution and number of the sensory papillæ serve for characterizing the larger and smaller groups of Nematodes.

The excretory organs of the Nematodes are variable. In a great many cases the apparatus is symmetrical, and consists of a vessel commencing in the posterior extremity in each lateral line (fig. 260), and passing anteriorly. In the vicinity of the anterior extremity both tubes pass out of the lateral lines, bend ventrally, and, in the median ventral line, unite into a short vesicle formed by an ectodermal cell—the cavity of which is lined by a continuation of the cuticle of the body—which opens into the excretory pore (fig. 263, Exp.). Asymmetry is occasioned through the excretory duct proceeding from the ventral pore to the lateral line, and it here proceeds as (or takes up) the left excretory canal, which anteriorly is a broader tube and runs along the left lateral line; shortly before its union with the excretory duct it throws out a branch to the right towards the lateral line, which, however, always remains weak, and runs posteriorly in the right lateral line; a few smaller branches in addition spring from the left main stem. In other species the right branch is completely suppressed; the entire organ thus lies in the left lateral line, and consists of the excretory duct, which occasionally opens quite in front near the lips, as well as the excretory canal, which throws out a number of lateral branches.

This excretory vesicle is a single elongated or horse-shoe-shaped cell, with a large nucleus and an intracellular tubular system, which is connected with the excretory duct arising from the excretory pore on the outer surface (fig. ). The so-called ventral gland is the only excretory organ of marine Nematodes, and probably represents a primitive form. Goldschmidt, who has investigated the excretory apparatus of Ascaris lumbricoides, considers that the vessels running in the lateral lines are only ducts to which belong a glandular system hitherto overlooked or otherwise interpreted. This system also lies in the lateral lines, and takes the form of two glandular tracts, forming a syncytial tissue in which lie the ducts, one dorsal, one ventral. In parts these tracts are connected by commissures, although their junction with the excretory vessels cannot be clearly made out. These statements, however, require confirmation. The author has further found that the anterior ends of the lateral canals, directly before they bend ventrally, anastomose with one another and give off anteriorly a small blind process, which can be interpreted as a rudiment of a canal coming from the head end, and as a matter of fact, according to Golowin, such anterior excretory canals exist in a number of genera.

In a number of Nematodes (Cheiracanthus, Capillaria, Trichocephalus, Trichinella, etc.), however, special excretory organs are lacking; possibly the cutaneous glands, which are in these species generally powerfully developed, replace these organs.

Sexual organs.—With the exception of a few species, the Nematodes are sexually differentiated.

(a) Female Sexual Organs.—The sexual orifice (vulva), surrounded by thick labia, is, as a rule, ventral and varies in position from near the head to near the anus. It leads into a short or long vagina (ectodermic), bifurcating into the two uteri, which may be long or short; the long filiform ovaries are continuations of them (fig. 264). Further there is often, e.g., in Ankylostoma, a differentiation into the following parts: (1) Ovejector: the specialized portion of the uterus before it joins the vagina; there may be a separate one for each uterus, or a common one for both uteri. (2) Seminal receptacle: at the other extremity of the uterus. (3) Oviduct: a narrow tube connecting the ovary with the uterus proper. (For the explanation of the terms convergent and divergent uteri vide footnote p. .) Uterus and ovaries, which arise in the first place from a single cell, lie between the body wall and the gut and are surrounded by connective tissue. In some species (for instance, Trichinella) the ovary is single.

At the blind end of the ovary there is a mass of protoplasm with numerous nuclei that multiply continuously. Gradually the nuclei arrange themselves in longitudinal rows (fig. 265) and the protoplasm commences to leave the periphery and surround each nucleus. The nearer to the uterus the more progressive is this loosening process, until club-shaped cells each containing a nucleus are developed. The most pointed end of each, however, is still attached to an axial fibre of protoplasm, the rhachis; probably this has some connection with the nutrition of the ova. Finally the ova fall off and reach the uterus, where they are fertilized and enclosed in shells.

(b) Male Sexual Organs.—There is never more than one testis (fig. 266), which is a straight or sinuous tube of the same construction as an ovary, and in which the mother cells originate in the same manner as the ova. In the same way as the ovary passes into the uterus, so does the testis pass into the spermatic duct; the latter is often divided into the somewhat dilated seminal vesicle and into the muscular ductus ejaculatorius, which, running ventral to the intestine backward (fig. 267), finally opens into the cloaca. In many species, e.g., A. duodenale, the ejaculatory duct is surrounded for a greater or less portion of its extent by the cement gland, the secretion of which (brownish or blackish in colour) serves for copulation. The ejaculatory duct of the large Ascaridæ is for the most of its course surrounded by a muscular network which takes its origin from the two dilator cells of the gut (fig. 268 F.). The spermatozoa of the Nematodes, it may be noted, only attain their full development after the sperm mother cells have been conveyed by copulation into the uteri of the female genitalia. In their form (sheathless, capable of amœboid motion) they differ from those of most other animals.

Spicules.—The male genital apparatus is also provided with one or two sacs, situated on the dorsal side of the intestine, and opening into the cloaca. In each sac there is a chitinous rod-like body, the spicule. Further, in many cases there exists, more or less fixed in the dorsal wall of the cloaca, a chitinous structure, the accessory piece or gubernaculum, the latter name implying its function of guiding the spicules during copulation (fig. 264A). A special muscular apparatus, consisting of protractors and retractors, moves the spicules. The protractors or exsertors in the large Ascaridæ consist of four flat band-like muscles which surround the spicule sac. Two long muscle cells which arise proportionally far forward on the dorsal side of the lateral line and are inserted into the base of the spicules serve as retractors. The spicules can be projected from the cloacal orifice (anus) during copulation, and when they are introduced into the vagina they serve as prehensile organs, perhaps also as stimulatory organs.

Bursa copulatrix.—The males in many genera possess epidermal wing-like appendages at their posterior extremity. These are supported by elongated tactile papillæ called ribs. In the most highly developed bursæ, e.g., in the Strongylidæ, the ribs are called rays, as they consist not only of nerve fibres but mainly of “pulp,” i.e., prolongations of the subcuticular layer. Bursæ are either open, i.e., bilaterally symmetrical, or closed, when the posterior border is continuous all round. A pseudo-bursa is one unsupported by ribs or rays, e.g., in Trichuris. The bursa serves as an organ of prehension during copulation. Some forms, moreover, carry a sucker at the posterior extremity (e.g., Heterakis); in others the spicules and other prehensile organs are absent; they are then replaced by an evertible cloaca, e.g., Trichinella.