PARASITISM AND DEGENERATION 181 
like that of the external and internal parasites, is simply a 
matter of convenience, and does not indicate at all any 
blood relationship among the members of any one group. 
95. The simple structure of parasites—In all cases the 
body of a parasite is simpler in structure than the body of 
other animals which are closely related to the parasite— 
that is, animals that live parasitically have simpler bodies 
than animals that live free active lives, competing for 
food with the other animals about them. This simplicity 
is not primitive, but results from the loss or atrophy of the 
structures which the mode of life renders useless. Many 
parasites are attached firmly to their host, and do not move 
about. They have no need of the power of locomotion. 
They are carried by their host. Such parasites are usually 
without wings, legs, or other locomotory organs. Because 
they have given up locomotion they have no need of or- 
gans 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 detection of food or of enemies. Because these im- 
portant organs, which depend for their successful activity 
on a highly organized nervous system, are lacking, the 
nervous system of parasites is usually very simple and un- 
developed. Again, because the parasite usually has for 
its sustenance the already 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 circu- 
latory organs; so that most fixed parasites have these sys- 
tems of organs in simple condition. Altogether the body 
of a fixed, permanent parasite is so simplified and so want- 
13 
