PARASITISM, ETC., ON THE SEA-SHORE 145 



found to sit on the middle of the bivalve with a pair of the 

 bivalve's gills on each side of them, and in such a position 

 as to bring the edge of a gill-leaflet up against the abdomen, 

 the smaller crabs being found in any position, but most 

 often upon the gill. The position taken up by the crab 

 enables it to feed without trouble by scraping its claws over 

 the gill and transferring to its mouth the mucous strings of 

 food which are in process of being wafted along to feed the 

 mussel. In view of the way in which the crab robs the 

 mussel of its food and of the manner in which it is adapted 

 for that purpose we are justified in regarding it as a parasite, 

 although its effects upon the host are not of a serious nature. 

 Orton's researches, which at the time of writing are not 

 yet complete, bring out several facts of interest in connection 

 with the Ufe-history. Thus the females appear to be 

 dimorphic, since they are male-like when still very small and 

 only assume the adult female characters after insemination, 

 which occurs at a very early stage. The male apparently 

 visits the female after the latter has taken up its abode in 

 the mussel, since male crabs have been found trapped 

 between the shell-valves on their way in. The extremely 

 tough carapace of the pea-crab is regarded as adaptive since 

 it prevents the animal being crushed to death when caught 

 in the way described. 



Turning to the smaller members of the Crustacea 

 (Entomostraca) we find the parasitic Copepoda passing the 

 greater part of their lives as parasites upon fish and other 

 animals. They often occur in great numbers upon any 

 particular host and may assume strange forms, many losing 

 almost all resemblance to their free-living relatives. Thus, 

 they may be worm-like in appearance and show other 

 profound structural modifications. Some bore into the 

 tissues of their host, while others fix themselves to the gills 

 or elsewhere. All these parasitic Copepods are free-swimming 

 in their early stages. In some, the male, though highly 

 organised, is very small and lives what is practically a 

 parasitic life on the body of the female. (See Marsh, C. D., 

 in Ward and Whipple, op. cit.) 



