When various parasites are examined it will be noted that practically in all cases the body of a parasite is simpler in structure than the body of other animals closely related to it; that is, species which live parasitically, obtaining their food from and being carried about by a host, have simpler bodies than related forms that live free active lives, competing for food with other animals about them. This simplicity is not primitive, but results from the loss or atrophy of the structures which the special mode of life of the parasite renders useless. Many parasites are attached firmly by hooks or suckers to their host, and do not move about independently of it. They have no need of the power of locomotion, and accordingly are usually without wings, legs, or other locomotory organs. Because they have no need of locomotion they have no need of organs of orientation, those special sense organs like the eyes, ears, and feelers which serve to guide and direct the moving animal; and most fixed parasites will be found to have no eyes, or any of those organs accessory to locomotion, and which serve for the detection of food or of enemies. Because these important organs, which depend for their successful activity on a well-organized nervous system, are lacking, the nervous system of parasites is usually very simple. Again, because the parasite usually feeds on the already digested food or the blood of its host, most parasites have a very simple alimentary canal, or even none at all. Finally, as the fixed parasite leads a wholly sedentary and inactive life, the breaking down and rebuilding of tissue in its body goes on very slowly and in minimum degree, so that there is little need of highly developed respiratory and circulatory systems; and most fixed and internal parasites have these systems of organs decidedly simplified. Altogether the body of a fixed permanent parasite is so simplified and so wanting in all those special structures which characterize the active, complex animals that it often presents a very different appearance from those forms with which we know it to be nearly related. This simplicity due to loss or reduction of parts is called degeneration. Such simplicity of body-structure due to degeneration is, however, essentially different in its origin from the simplicity of the lower simpler animals. In them the simplicity of body is primitive; they are generalized animals; the simplicity of degeneration is acquired; it is really an adaptation, or specialization.

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.

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.

Fig. 160.—Larva of a sphinx-moth,
with cocoons of a parasitic
ichneumon fly. (From
specimen.)