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.