282 NEMATHELMINTHES. 



as numerous generations of the free Rhabditis form may succeed 

 one another, before there is a return to the parasitic condition. The 

 Rliabdonema appendiculata is also peculiar, in that the form parasitic 

 in the slug is a larva characterized by the absence of a mouth 

 and by the possession of two long band-shaped caudal appendages ; 

 it quickly attains maturity, but only after a migration into damp 

 earth and after losing the caudal appendages and casting the skin. 



The Nematoda feed on organic juices, some of them also on blood, 

 and are enabled by their armed mouth to inflict wounds and to gnaw 

 tissues. They move by bending their body with a rapid undulatory 

 movement towards the ventral and dorsal surfaces. Although most 

 Nematoda are parasitic, they usually lead an independent life in 

 certain stages of their life-history. Numerous small Nematoda 

 (Anguillulidae, JEnoplidae), however, are never parasitic, but live 

 freely in fresh and salt water, and in the earth. Some Nematodes 

 are parasitic in plants, e.y., Anguillula tritici, dipsaci, etc., and may 

 even produce gall-like deformities (Tylenclius), others live in decaying 

 vegetable matter, e.g., the vinegar worm in fermenting vinegar and 

 paste. In many cases the migration of the parasite is a condition 

 necessary to the attainment of sexual maturity, e.g., in Mermis, where 

 sexual organs are not developed till the worm leaves its host and 

 becomes free in damp earth, in which copulation of the sexes is 

 effected. Finally there are certain small Nematodes the females of 

 which alone are parasitic. These, after copulation in the free state 

 with the small males, migrate into insects, and under the favourable 

 conditions of parasitism not only increase enormously in size, but also 

 undergo structural modifications favourable for the production of a 

 large number of embryos. In AUractonema gibbosum and in Sphae- 

 rularia bombi, the remarkable parasite of the humble-bee, the females, 

 after copulating in the free state, migrate, the former into the larva 

 of the gall-fly Cecidomyia pini, the latter into the queen-bees, which 

 live through the winter. Here the gut degenerates, and a kind of 

 hernia of the body-wall, containing the generative organs, is formed, 

 while the body of the worm shrinks to a small appendage (Fig. 231). 

 The eggs develop in the body of the insect into larvae, which pass 

 out of the body, become free, and, either after a few days (Attract- 

 onema) or after some months (Spliaerularia}, become sexually 

 mature. 



The power possessed by small Neniatodes of resisting the effects 

 of prolonged desiccation and of coming to life again (so to speak) 

 on being moistened is remarkable. 



