338 ANIMAL STUDIES 



notheres) which live habitually inside the shells of living 

 mussels. The mussels and the crabs live together in per- 

 fect harmony and to their mutual benefit. 



PIG. 203. Hermit-crab (Pagurus) in shell, with a sea-anemone (Adamsia palliate^ 

 attached to the shell. After HERTWIG. 



272. Relation of parasite and host. There are many 

 instances in the animal kingdom of an association between 

 two animals by which one gains advantages great or small, 

 sometimes even obtaining all the necessities of life, while 

 the other gains nothing, but suffers corresponding dis- 

 advantage, often even the loss of life itself. This is the 

 association between two animals whereby one, the para- 

 site, lives on or in the other, the host, and at the ex- 

 pense of the host. Parasitism is a common phenomenon 

 in all groups of animals; but parasites themselves are 

 mostly invertebrates. When an animal can get along 

 more safely or more easily by living at the expense of 

 some other animal and takes up such a life, it becomes a 

 parasite. 



273. Kinds of parasitism. The bird-lice (Mallophaga), 

 which infest the bodies of all kinds of birds and are found 



