Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/339

Rh N E M A T O I D E A 325 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 Anguillulidae. 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 belong ing to the free-living genus Anguilhda 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 ; J (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 Invertebrata are usually immature forms which attain their full develop ment in the body of some vertebrate ; but there are a number of species which in the sexually adult condition are peculiar to the Invertebrata.^ The Nematoidea 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 &quot;intermediate&quot; to a &quot;definitive&quot; 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 &quot;hairworms&quot; Gordius and Mcrmis. The adult Gordius aquations inhabits clear running water; it is a long slender worm often about a foot in length but only -jV inch in diameter. Several individuals are frequently found together twisted into a knot, whence the name Gordius. The 1 Ercolani successfully cultivated Oxyuris curwla, Strongylus armatus, and other species in damp earth ; the free generation was found to differ from the parasitic by its small size, and by the females being ovoviviparous instead of oviparous. To this phenomenon he gave the name of dimorphobiosis. 2 The genera Ascaris, Filaria, Trichosoma are found throughout the Vertebrata ; C ucullanus (in the adult condition) only in fishes and Amphibia; Dochmius, Trichocephalus, Trichina, and Pseudaliiis live only in the Mammalia, the last-mentioned genus being confined to the order Cetacea ; Strongylus and Physaloptera are peculiar to mammals, birds, and reptiles, while Dispharagvs, Syngamus, and Hystrichis are confined to birds. Gordius and Mermis (in the larval state) are with one or two exceptions confined to the Invertebrata and Sphserularia to bees. Oxyuris, though chiefly parasitic in the ^^ammalia, occurs also in reptiles, Amphibia, and one or two insects. Dacnitis and Ichthyo- neina are only found in fishes. larva when first hatched is provided with a number of cephalic hooks by the aid of which it bores its way into the larvse of the gnat and other Diptcra ; there it becomes encysted, but con tinues 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 free and finds its way through the alimentary canal to the exterior ; the cephalic armature disappears, the alimentary canal be comes rudimentary, and after acquiring sexual organs the larva assumes the character of theadult (Villot). (2) To the second class be long 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 gemis 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 genera tions which regularly alternate. The worm inhabits the lung of the frog and toad, and is hermaphrodite (Schneider) or partheno- genetic (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 arc 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 under go 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 medinensis, 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 medi- ncnsis the Guinea worm is parasitic in the subcutaneous connec tive tissiie of man (occasionally also in the horse). It is chiefly found in the tropical parts of Asia and Africa, but 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 un dergo 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 faeces, and the young become fully formed within the egg, and when accidentally swal lowed by their host are liberated by the solvent action of the gastric juice and com plete their development. This simple type of life history has been experimentally proved by Leuckart to be characteristic of Trichocephalus affinis, Oxyuris ambigua, and other species. (2) The life history of Ollulanus tricuspis is an example of the second class. Ollulanus tri cuspis is found in the adult state in the ali mentary 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 woim degenerates and loses all trace of struc ture. 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 ex crement. Leuckart proved experimentally that these young forms become encysted in the muscles of mice, and the cycle is com pleted after the mouse is devoured by a cat. FIG. 2. Trichina en cysted among mus cular fibres. After Leuckart. The well-known Trichina spiralis (fig. 2) has a life history closely resembling that