ANIMAL PARASITES AND DEGENERATION 413 



have given up locomotion they have no need of organs of 

 orientation, those special sense organs like eyes and ears 

 and feelers which serve to guide and direct the moving 

 animal; and most non-locomotory parasites will be found 

 to have no eyes, nor any of the organs of special sense which 

 are accessory to locomotion, and which serve for the de- 

 tection of food or enemies. Because these important organs, 

 which depend for their successful activity on a highly or- 

 ganized nervous system, are lacking, the nervous system 

 of parasites is usually very simple and undeveloped. Again, 

 because the parasite usually has for its sustenance the al- 

 ready digested highly nutritious food elaborated by its host, 

 most parasites have a very simple alimentary canal, or even 

 no alimentary canal at all. Finally, as the fixed parasite 

 leads a wholly sedentary and inactive life, the breaking down 

 and rebuilding of tissue in its body go on very slowly and in 

 minimum degree, and there is no need of highly developed 

 respiratory and circulatory organs, so that most fixed para- 

 sites have these systems of organs in simple condition. 

 Altogether the body of a fixed, permanent parasite is so 

 simplified and so wanting in all those special structures 

 which characterize the higher, active, complex animals, 

 that it often presents a very different appearance from those 

 animals with which we know it to be nearly related. 



Internal parasites. — Inside the body of most animals 

 live various parasites belonging to the great branch of worms. 

 The tapeworm and the deadly trichina (fig. 206; for ac- 

 count see p. 147) are conspicuous examples of these. The 

 tapeworm (fig. 207) has the form of a narrow ribbon, per- 

 haps several yards long, attached at one end to the wall of 

 the intestine, while the remainder hangs freely in the interior. 

 Its body is composed of segments or serially arranged parts, 

 of which there are about 850 altogether. It has no 

 mouth or stomach. It feeds simply by absorbing into its 

 body, through the skin, the nutritious food already partly 



