PAKASITISiM 



833 



as fish lice, live as parasites attached to the gills or ex- 

 ternal surface of fish and other aquatic forms. Of these the 

 Lerneans are most remarkable in respect to degeneration. 

 They occur on all aquatic creatures from the coelenterates 

 and echinoderms to the whales. Out of a dozen codfish 

 I examined on the Maine coast, eight contained one or 



FlG. 377. Nauplius or larva 

 ofLernea branchialis. Pho- 

 tographed through the micro- 

 scope. 



FIG. 378. Adult female Ler- 

 nea branchialis. The 

 branched process at the left 

 enables it to hold fast in the 

 gills. Photograph life size. 



more individuals of Lernea branchialis embedded in the 

 gills. The male of this species leads an independent life. 

 This brief discussion of parasites shows that but few, 

 if any, of the species above the protozoa are exempt from 

 having their bodies made the feeding ground for smaller 

 creatures. If certain species of insects occur in vast 

 numbers one season, few will be present the following 

 year owing to destruction by parasites. The life of these 

 paupers means death to many of their hosts, and so large a 

 part do they play in the world of physical life that Dr. 

 Smith says, " Without them there would be no possibility 

 of an existing vegetation." 



