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 Ccelentera, 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 larvae 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 



