200 SECRETS OF ANIMAL LIFE 
host. There are partial parasites which spend a 
chapter or two of their life in freedom, and there 
are complete parasites which pass from host to host 
in a never-broken vicious circle. In proportion to 
the intimacy of the dependehce is the degeneration 
of the parasite, which affects especially the sensory, 
nervous, muscular, and alimentary systems. The 
reproductive system, on the other hand, is often 
highly developed and the multiplication very pro- 
lific. This may be correlated primarily with the 
abundance of stimulating food available without 
exertion, and secondarily with the enormous chances 
of death in the life-history. For most of the 
parasites owe their survival to being many, not to 
being strong. The intricacies in the life-histories 
are often extraordinary, and are due in part to the 
fact that the parasite has to share in the knots in 
which their hosts are involved in the web of life, 
for it is natural enough that the bladderworm of 
the mouse should become the tapeworm of the cat. 
Ugly parasites are common, but many are conspicu- 
ously well adapted. Thus the tapeworm absorbs 
food by the whole surface of its body; it is fixed 
to its host by muscular adhesive suckers and often 
by attaching hooks as well; it can thrive with a 
minimum of oxygen; it has a mysterious “ anti- 
body” which saves it from being digested in its 
host’s intestine; it produces millions of eggs which 
it is able to fertilize of itself. It may be repulsive, 
but in the technical biological sense, relative to 
given conditions, it is “ fit.” 
