[Illustration: FIG. 81.—Female _Desmoscolex elongatus_ Panceri, ventral view, × 260. _a_, Ovary. (From Panceri.)] _Trichoderma oxycaudatum_ Greef[201] is a minute animal, 0.3 mm. long, which has no head or ventral spines, but whose body is ringed and covered with long hair-like bristles. The male has two spicules, and the internal organisation recalls that of other Nematodes; still its ringed body has induced some authorities to place it near to _Desmoscolex_. THE LIFE-HISTORY OF NEMATODES. Although, considering the enormous number of species of Nematodes and the remarkable diversity of the conditions under {160}which they live, their bodily structure shows a very striking uniformity, the same is by no means the case with their life-history, which exhibits an astounding variety. Von Linstow[202] has arranged the various modifications, which occur under fourteen heads. He includes in his list the Gordian worms, which we have placed under a different heading. The following account has been taken from his paper, with a few alterations:— 1. The embryos develop, with a larval stage and without any change of medium, directly into the mature sexual forms. They live in fresh, brackish, or salt water, in plants, in the earth or in decaying organic matter: examples, _Dorylaimus_, _Enoplus_, _Plectus_, _Monhystera_. 2. The larvae live in the earth, the sexual forms in plants: examples, _Tylenchus tritici_ and _T. devastatrix_, _Heterodera schachtii_ (Figs. 77 and 78). 3. The larvae live in animals, after whose death and decay they are set free and develop into the sexual animals in the earth: example, _Rhabditis pellio_. 4. The bisexual forms live in the earth, and the fertilised females bore into animals (insects), and here produce embryos: example, _Sphaerularia bombi_ (Fig. 76). 5. The bisexual forms live in the earth; the females do not develop, but the males make their way into Insects (Beetles), and becoming hermaphrodite, develop ova which give rise to the bisexual form: example, _Bradynema rigidum_. 6. The larvae live in the earth, the sexual form in Vertebrates: examples, _Dochmius_, _Strongylus_. 7. The Nematode lives as a hermaphrodite in animals, the offspring of this, by an alternation of generations, become sexual in the earth: example, _Rhabdonema_ in Frog. 8. A bisexual free form gives origin to a bisexual parasitic form living in an animal: example, _Leptodera appendiculata_ in Snails. 9. The eggs develop in the earth, and give rise to embryos which are transferred whilst still in the egg-cell to the body of an animal. The embryos hatch out and form bisexual parasites: examples, _Oxyuris_, _Trichocephalus_. 10. The larvae live in insects, the sexual worms in water or in the earth: example, _Mermis_. 11. The larva lives encapsuled and is passively transferred to {161}a second animal: examples, _Ollulanus_, from Mouse to Cat; _Cucullanus elegans_, from _Cyclops_ to Perch; _Spiroptera obtusa_, from Meal-worm to Mouse.