NEMATOIDEA. The name .Arentatoidea (vc) ,ua, thread, €1-603, form) was first introduced by Rudolphi, but the group had been previously recognized as distinct by Zeder under the name of Ascarides. They are now by many systematists united with the Acantleocepleali to form the order Nemathelminthes.

The Nematoidea possess an elongated and thread-like form (see fig. 1), varying in length from a few lines up to several feet. The body is covered externally by a chitinous cuticle which is a product of the subjacent epidermic layer; this cuticle is frequently prolonged into spines and papillm, which are especially developed at the anterior end of the body. The mouth opens at one extremity of the body and the anus at or near the other. Beneath the epidermis is a longitudinal layer of muscle-fibres which are separated into four distinct groups by the dorsal, ventral, and lateral areas ; these are occupied by a continuation of the epidermic layer; in the lateral areas run two thin-walled tubes with clear contents, which unite in the anterior part of the body and open by a pore situated on the ventral area. These vessels are considered to represent the segmental organs of other worms. The lateral areas are entirely absent in Gordius. The body cavity is largely occupied by connective tissue, and neither here nor elsewhere are cilia found at any period of development. The alimentary tract consists of a straight tube running from the mouth to the anus without any convolutions ; it is separable into three divisions : - a muscular oesophagus, which is often provided with cuticular teeth ; a cellular intestine ; and (3) a short terminal rectum surrounded by muscular fibres.

A nervous system has been shown to exist in many species, and consists of a pericesophageal ring giving off several nerves which run forwards and backwards along the lateral and median lines. Some of the free-living forms possess eye specks. The sexes are distinct (with the exception of a few forms that are hermaphrodite), and the male is always smaller than the female.

The generative organs consist of one or two tubes, in the upper portion of which the ova or spermatozoa are developed, the lower portion serving as an oviduct or vas deferens ' • the female generative organs open at the middle of the body, the male close to the posterior extremity into the terminal portion of the alimentary canal ; from this cloaca a diverticulum is given off in which are developed one to three chitinous spicules that subserve the function of copulation. The spermatozoa differ from those of other animals in having the form of cells which sometimes perform amoeboid movements. The development is similar to that of the ova, with which they appear to be homologous.

Mode of Life and Metamorphoses. - While the majority of the Nematodes are parasites, there are many that are never dir any period of their life parasitic. These free-living forms are found everywhere - in salt and fresh water, in damp earth and moss, and among decaying substances ; they are always minute in size, and like many other lower forms of life, are capable of retaining their vitality for a long period even when dried, which accounts for their wide distribution ; this faculty is also possessed by certain of the parasitic Nematodes, especially by those which lead a free existence during a part of their life-cycle. The free-living differ from the majority of the parasitic forms in undergoing no metamorphosis ; they also possess certain structural peculiarities which led Bastian (Trans. Linn. Soc., 1865) to separate them into a distinct family, the Anguillulidx. It is impossible, however, to draw a strict line of demarcation between the free and parasitic species, since - (1) many of the so-called free Nematoidea live in the slime of molluscs (Villot), and are therefore really parasitic ; (2) while certain species belonging to the free-living genus Anguillula are normally parasitic (e.g., A. tritici, which lives encysted in ears of wheat), other species occasionally adopt the parasitic mode of existence, and become encysted in slugs, snails, &c. ; (3) it has been experimentally proved that many normally parasitic genera are capable of leading a free existence ;1 (4) transitional forms exist which are free at one period of their life and parasitic at another. The parasitic Nematodes include by far the greatest number of the known genera ; they are found in nearly all the orders of the animal kingdom, but more especially among the Vertebrata, and of these the Mammalia are infested by a greater variety than any of the other groups. No less than nineteen distinct species have been described as occurring in man. The Nematode parasites of the Invertebrate are usually immature forms which attain their full development in the body of some vertebrate ; but there are a number of species which in the sexually adult condition are peculiar to the Invertebrata.2 The Nenzatoidea contain about as many parasitic species as all the other groups of internal parasites taken together ; they are found in almost all the organs of the body, and by their presence, especially when encysted in the tissues and during their migration from one part of the body to another, give rise to various pathological conditions. Although some attain their full development in the body of a single host - in this respect differing from all other Entozoa - the majority do not become sexually mature until after their transference from an " intermediate " to a " definitive " host. This migration is usually accompanied by a more or less complete metamorphosis, which is, however, not so conspicuous as in most other parasites, e.g., the Trematoda. In some cases (many species of Ascaris) the metamorphosis is reduced to a simple process of growth.

The parasitic and free-living Nematodes are connected by transitional forms which are free at one stage of their existence and parasitic at another ; they may be divided into two classes - those that are parasitic in the larval state but free when adult, and those that are free in the larval state but parasitic when adult.

(1) To the first class belong the " ha irworms " Gordius and .zifermis. The adult Gordizcs aquaticus inhabits clear running water; it is a long slender worm often about a foot in length but only Ay inch in diameter. Several individuals are frequently found together twisted into a knot, whence the name Gordius. The larva when first hatched is provided with a nnmber of cephalic hooks by the aid of which it bores its way into the larva; of the gnat and other Diptcra ; there it becomes encysted, but continues to move about within the cyst. The gnat larvae are devoured by fish, and the young Gordius is set free and penetrates the mucous membrane of the intestine, where it encysts itself and becomes quiescent; in this second larval period the cyst differs in character from that formed during the first larval period. In the spring, about six months after the second encystment, the larva becomes froo and finds its way through the alimentary canal to the exterior ; the cephalic armature disappears, the alimentary canal becomes rudimentary, and after acquiring sexual organs the larva assumes the character of the adult (V ill ot). (2) To the second class belong Dochmius, Strongylus, and many species of Ascaris ; the embryo on leaving the egg lives free in water or damp earth, and resembles very closely the free-living genus Rhabditis. After a longer or shorter period it enters the alimentary canal of its proper host and becomes sexually mature. Ascaris nigrovenosa has a developmental history which is entirely anomalous, passing through two sexual generations which regularly alternate. The worm inhabits the lung of the frog and toad, and is hermaphrodite (Schneider) or parthenogenetic (Leuckart) ; the embryos hatched from the eggs find their way through the lungs into the alimentary canal and thence to the exterior ; in a few days they develop into a sexual Rhabditis, in which the sexes are distinct ; the eggs remain within the uterus, and the young when hatched break through its walls and live free in the perivisceral cavity of the mother, devouring the organs of the body until only the outer cuticle is left ; this eventually breaks and sets free the young, which are without teeth, and have therefore lost the typical Rhabditis form. They live for some time in water or mud, occasionally entering the bodies of water snails, but undergo no change until they reach the lung of a frog, when the cycle begins anew. Although several species belonging to the second class occasionally enter the bodies of water snails and other animals before reaching their definitive host, they undergo no alteration of form in this intermediate host ; the case is different, however, in Filaria meclinensis, F. bancrofti, and other forms, in which a free larval is followed by a parasitic existence in two distinct hosts, all the changes being accompanied by a metamorphosis. Filaria mcdinensis - the Guinea worm - is parasitic in the subcutaneous connective tissue of man (occasionally also in the horse). It is chiefly found in the tropical parts of Asia and Africa, hut has also been met with in South Carolina and several of the West Indian islands. The adult worm, which sometimes reaches a length of 6 feet, has the characters of a female, but is probably hermaphrodite, seeing that no males have ever been discovered. It is viviparous, and the young, which, unlike the parent, are provided with a long tail, live free in water; it was formerly believed from the frequency with which the legs and feet were attacked by this parasite that the embryo entered the skin directly from the water, but it has been proved by Fedschenko that the larva bores its way into the body of a Cyclops and there undergoes further development. It is probable that the parasite is then transferred to the alimentary canal of man by means of drinking water, and thence makes its way to the sub cutaneous connective tissue.

The Nematoidea which are parasitic during their whole life may similarly be divided into two classes - those which undergo their development in a single host, and those which undergo their development in the bodies of two distinct hosts (1) In the former class the eggs are extruded with the aces, and the young become fully formed within the eg6, and when accidentally swallowed by their host are liberated by the solvent action of the gastric juice and complete their development. This simple type of life history has been experimentally proved by Leuckart to be characteristic of Trichocephalus affinis, Oxyuris arnbigua, and other species. (2) The life history of 011ulanus tricuspis is an example of the second class. 011ulanus tricuspis is found in the adult state in the alimentary canal of the cat ; the young worms are hatched in the alimentary canal, and often wander into the body of their host and become encysted in the lungs, liver, and otl:er organs ; during the encystment the mum degenerates and loses all trace of structure. This wandering appears to be accidental, and to have nothing to do with the further evolution of the animal which takes place in those embryos which are voided with the excrement. Leuckart proved experimentally that these young forms become encysted in the muscles of mice, and the cycle is comof 0/Zit/amts. The adult worm, which is of extremely minute size, the male being only 8th and the females of an inch in length, inhabits the alimentary caual of man and many other carnivorous mammalia; the young bore their way into the tissues and become encysted in the muscles - within the muscle-bundles according to Leuckart, but in the connective tissue between them according to Chatin and others. The co-existence of the asexual encysted form and the sexually mature adult in the same host, exceptionally found in 011ulanus and other Nematodes, is the rule iu Trichina ; many of the embryos, however, are extruded with the faces, and complete the life cycle by reaching the alimentary canal of rats and swine which frequently devour human ordure. Swine become infested with Trichina in this way and also by eating the dead bodies of rats, and the parasite is conveyed to the body of man along with the flesh of " trichinized" swine.

Bibliography. - General Treatises : - Cobbold, Entozoa (London, 1864) and Parasites (London, 1578); Leuckart, Die menschlichen Parasiten, vol. B., Leipsic, 1876; Kilehenmeister, Die Parasiten des Afenschen, Lf. 3, 2d ed., Leipsle, 1S81 ; Chatin, La Trichine et la Trichinote, Paris, 1883. Systematic : - Goeze, Versueh einer Naturg. der Eingeweidewiirmer (Blankenburg, 1782), and Erster Nachtrag, &c., mit Annzerk. von Zeder (Leipsie, 1800) ; Rudolph!, Entozoorum historic naturalis, Ams•erdam, 1808; Dujardin, Histoire naturelle des helminthes, Paris, 1841; Diesing, Systema helminthum, Vienna, 1850; Bastian, "Monograph of Anguillulldse," Trans. Linn. Soc., 1865; Biltschli, "Beitriige zur Kenntn. d. freileb. Nematoden," Nom Act. Aced. Leip., 1873, and " Veber freileb. Nematoden," Abh. Seek. naturf. Gesells. Frankfurt, 1874; Villot, "Fanne helmInth. de la Bretagne," Arch. Zeal. Exp., 1875; De Man, " Onderz. over vrij in de Aarde levende Nematodcn," Tijdsh. d. Nederland. Dierk. Vereen, 1875 and 1876; V. Linstow, numerous papers in Arch. fur. Naturg., 1872-83. Anatomy and Development : - Besides the text-books of zoology, see Lubbock, " Sphaerularia bombi," Nat. Hist. Rev., 1861 and 1864 ; Eberth, Untersuch. fiber Nematoden, Leipsie, 1863; Schneider, Monographie des Nematoden, Leipsie, 1866; Bastian, "Anatomy and Physiology of Nernatoids," Phil. Trans., 1866 ; Villot, "Monographie des Dragonneaux," Arch. Zeal. Esp., 1874; Blitschli, "Zur Entwlek. Gesehiehte des Cueullanus elegans," Zeitsch. miss. Zoos., 187G; Ereolani, "Osservazioni sulfa vita fibers dell' Ascaris nmeulosa," Mem. Ac. Bologna, 1877. Distribution - V. Linstow, Compendium der Helminthologie, Hanover, 1878. (F. E. B.)