Cylindrical vermi-form Scolecids, sometimes parasitic, sometimes free; integument not ciliated; a well-developed alimentary canal, with a mouth and anus, suspended freely in a body-cavity; sexes distinct, or rarely united.

The Nematoda comprise the so-called "Thread-worms" and "Round-worms," and, as their various names imply, possess a rounded and worm-shaped body (fig. 120, a), sometimes of great length. The cuticle is chitinous and porous; and there is generally a distinct annulation, though no true segmentation exists. The alimentary system is well developed, the mouth being anterior, and usually furnished with papillae (fig. 120, c). The gullet opens into a muscular stomach, from which an intestine conducts to a ventrally or terminally placed anus; the whole digestive tube being freely suspended in a body-cavity, which is filled with a sparsely corpusculated fluid. There is a water-vascular system, composed of lateral tubes, which open on the surface by a ventrally - placed pore. The nervous system is in the form of a ganglionic ring, surrounding the gullet, and sending filaments backwards and forwards. The sexes are mostly distinct, the external openings of the reproductive organs being placed near the anus. The males are usually less frequently met with and of smaller size than the females, and they possess a single or double spicular penis (fig. 120, b). Metamorphosis may or may not occur during development.

As before said, most of the Nematoda are internal parasites, inhabiting the alimentary canal, the pulmonary tubes, or the areolar tissue, in man and in many other vertebrate animals; but a large section of the order are of a permanently free habit of existence.

Fig. 120.   Morphology of Nematoda. a A scaris lumbricoides, male, reduced in size ; b Hinder extremity of the same, with the reproductive spicula, enlarged; c Head of the same enlarged, showing the tubercles round the mouth ; d Ovum of the same, highly magnified, with the fully developed worm in its interior; e Male of Oxyuris vermicularis, five times the natural size ; f Female of the same, similarly enlarged ; g Male of the same, highly magnified ; h Embryo of Dracunculus, magnified 500 diameters ; i Embryos of the same, magnified 60 diameters; l A single Trichina, encapsuled in the muscles, highly magnified. (Chiefly after Leuckart, Spencer Cobbold, and Bastian.)

Fig. 120. - Morphology of Nematoda. a A scaris lumbricoides, male, reduced in size ; b Hinder extremity of the same, with the reproductive spicula, enlarged; c Head of the same enlarged, showing the tubercles round the mouth ; d Ovum of the same, highly magnified, with the fully-developed worm in its interior; e Male of Oxyuris vermicularis, five times the natural size ; f Female of the same, similarly enlarged ; g Male of the same, highly magnified ; h Embryo of Dracunculus, magnified 500 diameters ; i Embryos of the same, magnified 60 diameters; l A single Trichina, encapsuled in the muscles, highly magnified. (Chiefly after Leuckart, Spencer Cobbold, and Bastian.)

Amongst the more important members of the parasitic section of the Nematoda may be mentioned the Ascaris lumbricoides, the Oxyuris vermicularis, the Trichocephalus dispar, the Sclerostoma duodenale, the Dracunculus medinensis, and the Trichina spiralis.

The Ascaris lumbricoides, or common Round-worm, inhabits the intestine of man, and sometimes of other mammals, especially the pig, often attaining a length of several inches. The ova are expelled with the faeces, and the embryo is developed within the ovum prior to its rupture, but not till after the lapse of several months (fig. 120, d). When fully formed, the embryo is about one-hundredth of an inch in length, and its development is not exactly known, though it appears to be directly transferred from river or pond water to the alimentary canal of its host. The body of the adult (fig. 120, a) is cylindrical, attenuated at both extremities. At the anterior extremity is a sub-triangular mouth, surrounded by three tubercles (fig. 120, c). The anus is situated posteriorly. The females are larger than the males, and are much more numerous. These parasites are much more common in children than in adults, and they often occur in considerable numbers. They are usually found in the small intestine, though they sometimes wander into other situations.

The Oxyuris vermicularis, or "small Thread-worm" (fig. 120, e, f, g), is a gregarious worm, which inhabits the rectum, especially of children. It is the smallest of the intestinal worms of man, its average length not being more than a quarter of an inch, but the females are much bigger than the males.

The Trichocephalus dispar inhabits the caecum of man. It is from one and a half to two inches in length, and the anterior two-thirds of the body is extremely attenuated and thread-like.

The Sclerostoma duodenale inhabits the small intestine in the human subject, and is far from uncommon in Italy and in Egypt. It varies in size from one-third of an inch to half an inch, the females being the largest; and the symptoms to which it gives rise are often of a serious character.

The Trichina spiralis is a singular Nematoid, which gives rise to a painful and not uncommonly fatal train of symptoms, somewhat resembling rheumatic fever, and known as Trichiniasis. The Trichina is known in two different conditions, sexually immature or mature. In its sexually immature condition it inhabits the muscles, usually of the pig (the rat, however, being apparently its true host), in vast numbers, each worm (fig. 120, l) being coiled up in a little capsule or cyst. In this condition the worm is incapable of further development, and may remain, apparently for an indefinite period, without change, and without seeming to produce any injurious results to the animal affected. If, however, a portion of trichina-tous muscle be eaten by a warm-blooded vertebrate, and so introduced into the alimentary canal, an immediate development of young Trichinae is the result. The immature worms escape from their enveloping cysts, grow larger, develop sexual organs, and give birth to a numerous progeny, which they produce viviparously. The young Trichina: thus produced perforate the walls of the alimentary canal, and, after working their way amongst the muscles, become encysted. If the animal in which these changes go on has sufficient vitality to bear up under the severe symptoms which are produced by the migration of the Trichina, he is now safe; since they cannot become sexually mature, or develop themselves further, until again transferred to the alimentary canal of some other animal.

The Guinea-worm (Dracunculus or Filaria medinensis) is a Nematode worm, which inhabits, during one stage of its existence, the cellular tissue of the human body, generally attacking the legs, and often attaining a length of several feet. All known specimens of this parasite are impregnated females, containing a large number of young. The worm remains embedded in the body, in a more or less quiescent condition, for a year or more, at the end of which time it seeks the surface, in order to get rid of its young. No external aperture to the genital organs has hitherto been proved to exist, and it seems possible that the young are produced within the body of the parent by a process of internal gemmation. The young Filaria (fig. 120, h and i) consists of a vermiform body, terminating in a hair-like tail; and when set free from the parent, its further development probably takes place in water, when it is believed to be converted into one of the "Tank-worms" so common in India. In this condition, it is possible, as some believe, that sexual organs are developed, and that the females are impregnated. The young worm is believed to pass the intermediate portion of its life in the interior of fresh-water Crustaceans (Cyclops), but how it gains access to the cellular tissue of man is still unknown. According to Dr Bastian, however, it appears probable that the Guinea-worm " is a parasite only accidentally, and that it and its parents were originally free Nematoids."

In addition to the above-mentioned forms, it may be noted that minute parasitic Nematoids are not uncommonly found, sometimes in vast numbers, in the blood of various animals, including the clog, man himself, and various birds. Some of these haematozoa are embryonic, others appear to be mature, and they may or may not give rise by their presence to appreciable morbid symptoms. The origin of these haematozoa, their development, and the mode in which they are introduced into the blood, are subjects, for the most part, still shrouded in obscurity. The most remarkable species is the Filaria sanguinis-hominis, which in its immature state inhabits the blood of man in intertropical regions, its presence being commonly associated with chyluria, haematuria, and other morbid affections.

The second section of the Nematoda comprises worms which are not at any time parasitic, but which are permanently free. These "free Nematoids" (fig. 121) constitute the family of the Anguillulidae, of which about two hundred species have been already described, mostly inhabiting fresh water or the shores of the sea. They resemble the parasitic Nematoids in all the essential features of their anatomy, but they differ in often possessing pigment - spots, or rudimentary eyes, is being mostly provided with a terminal sucker, and in bringing forth comparatively few ova at a time ; the dangers to which the young are exposed being much less than in the parasitic forms. Amongst the more familiar free Nematoids are the Vinegar Eel (Anguillula aceti, fig. 122, A) and the Tylenchus (or Vibrio) tritici, which produces a sort of excrescence or gall upon the ear of wheat, causing the disease known to farmers as the "Purples," or "Ear Cockle."

The parasitic and free Nematoids are connected together by an Ascaris (A. nigrovenosa), which in succeeding generations is alternately free and parasitic. This Ascaris has long been known as inhabiting the lungs of the frog, but it has been shown by Meczniko w that "the young of this animal become real free Nematoids ; for, after passing from the intestine of the frog into damp earth or mud, they grow rapidly, and actually develop in the course of a few days, whilst still in this external medium, into sexually mature animals. Young, differing somewhat in external characters from their parents, are soon produced by them, and these attain merely a certain stage o development whilst in the moist earth, arriving at sexual maturity only after they have become parasites, and are ensconced in the lung of the frog" (Bastian). This extraordinary history is rendered still more remarkable, if it should be proved that the young of the parasitic forms of this Ascaris are produced by a process of parthenogenesis; and this seems to be highly probable, since none of the individuals which are found as parasites are males, but are universally females.

Fig. 121.   Free Nematoids. A, Anguillula aceti; B, Dory laimus stagnalis. Magnified. (After Bastian.)

Fig. 121. - Free Nematoids. A, Anguillula aceti; B, Dory-laimus stagnalis. Magnified. (After Bastian.)