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. Most remarkable sexual conditions are found to occur in the free-living genera Rhabditis and Diplogaster. While some of the species are bisexual, others are protandrous, self-fertilizing hermaphrodites. In cultures of the latter there occur very rare supplemental males which appear in no sense degenerate but as fit for reproduction as the males of the bisexual species. Though possessing a complete copulatory apparatus and producing large quantities of spermatozoa, they have lost their sexual instinct and play no part in the economy of the species. These “psychically decadent” individuals appear to represent the entire male sex of a bisexual species, and become unnecessary owing to the grafting of hermaphroditism on the female sex.
|
|
Mode of Life and Metamorphoses.—While the majority of the Nematodes are parasites, there are many that are never at 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 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 Nematoda 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. Some two dozen distinct species have been described as occurring in man. The Nematode parasites of the Invertebrata 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 Nematoda 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 so-called “hairworm,” Mermis, not to be confused with the Gordian worms.[3] The adult forms of M. nigrescens live in damp earth and may be seen after storms or early in the morning crawling up the stalks of plants, a fact which causes people to talk about showers of worms. The eggs are laid on
- ↑ Ercolani successfully cultivated Oxyuris curvula, 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.
- ↑ The genera Ascaris, Filaria, Trichosoma are found throughout the Vertebrata; Cucullanus (in the adult condition) only in fishes and Amphibia; Ankylostoma, Trichocephalus, Trichina and Pseudalius 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 Dispharagus, Syngamus and Hystrichis are confined to birds. Mermis (in the larval state) is confined to the Invertebrata and Sphaerularia to bees. Oxyuris, though chiefly parasitic in the Mammalia, occurs also in reptiles, Amphibia and one or two insects. Dacnitis and Ichthyonema are only found in fishes.
- ↑ See Nematomorpha.
