DISHARMONIES AND OTHER SHADOWS 579 



number of diverse parasites that may possess a lusty host, 

 with a wide range of appetite, we find that they are legion 

 like the demons. When we ask about the number of indi- 

 vidual parasites, it is beyond telling. 



(&) In many cases the association of parasites and host 

 is very specific, that is to say, many a parasite is only known 

 to occur in one definite kind of host, and many a host is 

 curiously non-susceptible to parasites not very different from 

 those which it harbours. The larvae of some of the fresh- 

 water mussels cannot become parasitic except on definite 

 species of fishes, though the larvae of some other kinds can 

 utilise many fishes. The larva of the liver-fluke in Britain 

 cannot develop except in one species of water-snail (Limncea 

 truncatula) , though in other countries other species some- 

 times serve. There are, however, some very cosmopolitan 

 parasites which occur in many hosts. 



(c) Parasitism is a relation of dependence always nutri- 

 tive, often more between the parasite and the host, but it 

 occurs in many grades. There are superficial ectoparasites 

 which often retain great activity, and intimate endoparasites 

 which may become practically part of their host. There 

 are partial parasites which retain independence during some 

 chapter or chapters of their life, and total parasites which 

 pass from host to host and are never free. Sometimes, it 

 is only the female that is parasitic, the male remaining free. 



(d) Corresponding to the degree of parasitism is the de- 

 generation of the parasite. This is sometimes to be witnessed 

 in the individual lifetime, e.g., in many Copepod Crus- 

 taceans where the young are free-living. In other cases it 

 may be inferred by comparing the parasite with related free- 

 living types. The retrogression affects especially the nervous, 

 sensory, muscular, and alimentary systems. The reproduc- 



