THE PROBLEM OF PARASITISM 199 
into the fishes caught on the fishermen’s deep-sea 
lines. There are no parasites among Echinoderms 
and few among Mollusks and Ceelentera, perhaps in 
part because the life of these types depends so much 
on the action of living lashes (cilia or flagella) in a 
fresh medium. Among plants most of the parasitic 
forms are fungi, and there are very few among 
flowering plants. But there is no getting away from 
the fact that parasitism is a very common mode of 
life. One of the European oaks harbors no fewer 
than ninety and nine different kinds of gall-flies, 
and the hundredth will probably have been dis- 
covered before this series of studies is published. 
The valuable Lac insect of India is beset by over 
thirty animal and vegetable parasites. The dog 
is a terrain for over forty; man and pig have 
far more. In short, no creature with a body is 
without a parasite, and the number that may pos- 
sess a lusty host with a wide range of appetite is 
legion. 
The association between parasite and host is often 
very specific; thus the larvee of some of the fresh- 
water mussels become temporary parasites on 
particular species of fishes and on no others, and 
the larva of the liver-fluke does not develop in 
Britain except within one particular kind of fresh- 
water snail. The relation of dependence—always 
nutritive, and often more—between parasite and 
host varies greatly in intimacy, for there are ex- 
ternal hangers-on, like fish-lice, and intimate 
endoparasites, which become almost part of their 
