189 ANIMAL LIFE 
ing 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. 
The simplicity of parasites does not indicate that they 
all belong to the groups of primitive simple animals. 
Parasitism is found in the whole range of animal life, 
from primitive to highest. ‘Their simplicity is something 
that has resulted from their mode of life. It is the result 
of a change in the body-structure which we can often 
trace in the development of the individual parasite. Many 
parasites in their young stages are free, active animals 
with a better or more complex body than they possess in 
their fully developed or adult stage. The simplicity of 
parasites is the result of degeneration—a degeneration 
that has been brought about by their adoption of a seden- 
tary, non-competitive parasitic life. And this simplicity of 
degeneration, and the simplicity of primitiveness should be 
sharply distinguished. Animals that are primitively simple 
have had only simple ancestors; animals that are simple 
by degeneration often have had highly organized, complex 
ancestors. And while in the life history or development of 
a primitively simple animal all the young stages are simpler 
than the adult, in a degenerate animal the young stages 
may be, and usually are, more complex and more highly 
organized than the adult stage. 
In the examples of parasitism that are described in 
the following pages all these general statements are illus- 
trated. 
96. Gregarina,—In the intestines of cray-fishes, centi- 
peds, and several kinds of insects may often be found 
certain one-celled animals (Protozoa) which are living as 
parasites. Their food, which they take into their minute 
body by absorption, is the intestinal fluids in which they 
lie. These parasitic Protozoa belong to the genus Grega- 
rina (Fig. 9) (see Chapter I). Because the body of any 
