The genus Ichthyonema is confined to fishes. The male is very minute and the female partly degenerate. It has no anus and no external opening to its generative organs. The uterus fills up almost the whole of the body-cavity. I. sanguineum Rud. is found encapsuled in the peritoneum of many fish.
Hystrichis and Dispharagus are confined to birds, where they occur in the oesophagus and stomach. Spiroptera reticulata Crep. occurs in horses, twisted in a spiral round tendons and muscles, forming tumours which require to be opened.
V. Family Mermithidae.
Nematodes without anus and with six mouth papillae. Two spicules in the males and three rows of numerous papillae.
Genera: Mermis, Bradynema, Atractonema, Allantonema, Sphaerularia, and others.
As a rule the Nematoda show but little trace of their parasitic mode of life, but in this family there is considerable degeneration, and in extreme cases the body of the female is reduced to a simple sac crowded with eggs. They are exclusively parasitic in insects. In some respects their structure shows a transition towards Nectonema and the Gordiidae; especially is this the case in the structure of their ventral nerve-cord.
The sexual form of Mermis nigrescens Duj.[[187]] lives in damp earth, and after storms and in the early morning is sometimes found in such numbers crawling up the stalks of plants, as to give rise to the popular idea that there has been a shower of worms. The male is unknown; the female lays her eggs in the ground, and there they hatch out. It is not known exactly how the larvae make their way into the grasshoppers in whose body-cavity they live, but in an allied species, M. albicans v. Sieb., the larvae have been observed boring their way into small caterpillars through their skin, and it seems probable that the larvae of M. nigrescens burrow in a similar way into young Orthoptera.
Bradynema rigidum Leuck.[[188]] is found in the adult stage living freely in the body-cavity of a small beetle Aphodius fimetarius, one of the Scarabeidae, from two to three to as many as thirty being found in one host, which does not seem much injured by their presence. The parasite is without mouth, anus, or excretory pore. The eggs hatch out in the uterus of the mother, and the larvae are male and female; they make their way into the body-cavity of the host, and here they pass an unusually long time, five months, soaking in osmotically the nutriment contained in the blood of the insect. Eventually they burrow through the walls of the intestine, and leaving the body of their host through the anus, find their way to the earth. Here, according to zur Strassen, the females die without playing any part in the perpetuation of the species. The males, on the other hand, having developed spermatozoa whilst in the larval stage (paedogenesis), afterwards form ova, and are in fact protandrous hermaphrodites, and become the mature parasites of the beetle, though how they enter the body of the host is unknown.
Fig. 74.—Allantonema mirabile Leuck. (From Leuckart.) A, Male Rhabditis stage, sexually mature, × 100; B, the mature female parasitic form, × 17, showing at the upper end part of the capsule richly supplied with the tracheae of the host, a beetle; C, female Rhabditis stage, sexually mature, × 100; D, the larva developed from the Rhabditis form, × 102.