Although, considering the enormous number of species of Nematodes and the remarkable diversity of the conditions under 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.