It is a common thing to find that parasites are either hermaphrodite or that the male is degenerate, as is the case with many of the parasitic Crustacea, but with one or two exceptions the Nematoda are bisexual, and although, as a rule, the males are smaller than the females, they show no other trace of degeneracy. In spite of the fact that the class as a whole shows but few special modifications consequent on a parasitic mode of life, it is clear that the Nematoda are peculiarly adapted for such a mode of life. Their elongated thread-like bodies afford little resistance to the passage of the food, which, as it passes through the intestine of the host, might tend to carry the parasites out of the body. At the same time their shape enables them to pierce and wriggle through the various tissues without making any very serious lesions such as might prove fatal to their host. Their extraordinary power of resisting desiccation both in the egg and in the adult state vastly increases their chances of ultimately hitting on the right host. They are capable of living in a state of suspended animation for months, and even years when dried (_vide_ p. 136), and of resuming their activity on being moistened. The great faculty this group shows for living parasitically is evinced by the extraordinary variety of life-history presented by the different species. There is scarcely a stage which may not be parasitic; the eggs, the larvae, the adults are all in some cases free, in others parasitic, and in many cases first the one and then the other. 2. OCCURRENCE AND EFFECT OF THE PARASITE ON THE HOST.—Von Linstow states that the only law that can be derived inductively from the study of the life-history of Nematodes is that those that live in animals never pass through all their stages of development in the same organ; consequently, in considering the distribution of {163}the parasites within the body of their host we have a double habitat to consider. Many forms, such as _Trichina spiralis_, wander from the intestine to the muscles; others, such as _Filaria medinensis_, from the alimentary canal to the lymphatics or blood vessels or subcutaneous tissues. Others pass from the body-cavity to the intestine, as the Mermithidae, which infest Insects, or from the stem and leaves of a plant to its flower, as in the case of _Tylenchus tritici_.