An excellent example of body degeneration due to the adoption of a parasitic habit is that of _Sacculina_ (fig. 159), a crustacean parasitic on other crustaceans, namely, crabs. The young _Sacculina_ is an active, free-swimming larva essentially like a young prawn or crab. After a short period of independent existence it attaches itself to the abdomen of a crab, and lives as a parasite. It completes its development under the influence of this parasitic life, and when adult bears absolutely no resemblance to such a typical crustacean as a crab or crayfish. Its body external to the host crab is simply a pulsating tumor-like sac, with no mouth-parts, no legs, and internally hardly any well-developed organs except those of reproduction. Degeneration here is carried very far. [Illustration: FIG. 159.--_Sacculina_, a parasitic crustacean; _A_, attached to a crab, the root-like processes of the parasite penetrating the body of the host; _B_, the active larval condition; _C_, the adult removed from its host. (After Haeckel.)] Various parasites have been referred to in Part II under their proper branch and class. The worms include an unusually large number of them, such as the tapeworms, trichinæ and other intestinal forms, all of which live as internal parasites in the alimentary canal or in other organs of higher animals, especially the vertebrates. Many crustaceans are parasitic, usually living, like the fish-lice, as fixed external parasites on fishes, other crustaceans, etc., but with a free and active larval stage. Among the insects, on the contrary, many of the parasitic forms (as the ichneumon flies) are free and active in the adult stage, but live as internal grubs or maggots in the larval stage. The ichneumon flies (of the order Hymenoptera) are four-winged, slender-bodied insects which lay their eggs either on or in (by means of a sharp piercing ovipositor) some caterpillar or beetle grub, into the body of which the young grub-like ichneumon larvæ burrow on hatching. The parasites feed on the body-tissues of the host, not attacking, however, such organs as the heart or nervous system, which would produce the immediate death of the host. The caterpillar lives with the ichneumon grubs within it usually until nearly time for its pupation. Often, indeed, it pupates with the parasite still in its body. But it never comes to maturity. The larval ichneumons pupate either within the body of its host, or in a tiny silken cocoons outside of its body (fig. 160). From the cocoons the winged adult ichneumons issue; and after mating the females find another caterpillar on whose body to lay their eggs. [Illustration: FIG. 160.--Larva of a sphinx-moth, with cocoons of a parasitic ichneumon fly. (From specimen.)]