144 THE BIOLOGY OF THE SEA-SHORE 



to make more. Generally speaking, the effects of this 

 parasitic castration of the crab accord with those observed 

 in the case of the castration of domestic animals by man, 

 though it should be noticed that the results in the second 

 case are due to the absence of a hormone. The effects of 

 Sacculina upon the female crab are less far-reaching, and 

 are merely those of speeding up the appearance of the adult 

 female characters and of partly destroying the ovary. 

 (Smith, op. cit., and Potts, 1906.) 



A remarkable example of a parasitic " chain " was pro- 

 vided by Perez's (1903) discovery of a hyper-parasite upon 

 a Sacculina which was parasitising a pea-crab (Pinnotheres), 

 the latter dwelling in a bivalve (Spondylus). 



Another crustacean, the pea-crab (Pinnotheres pisum), 

 inhabits the shells of various living bivalves, particularly the 

 mussel. This association has long been known, and is even 

 depicted in Egyptian hieroglyphics, where it has been inter- 

 preted as signifying the dependence of man upon his friends. 

 Recent researches, however, would seem to show that the 

 relation between crab and mussel is not that of partnership, 

 but rather that of parasite and host. 



Field (191 3) found that the stomach contents of the pea- 

 crab were composed of diatoms and other materials utilisable 

 by the mussels as food, and Wright (19 17) found that 

 Pinnotheres never (or at most, very rarely) occurred within 

 poorly nourished mussels, although it was frequent in those 

 from the fishing grounds, where the shell-fish were fat and 

 were making rapid growth. He concludes that the associa- 

 tion is a profitable one from the point of view of the crab. 

 In certain cases where the female had attained a relatively 

 large size it would seem to have exerted considerable 

 pressure upon the mantle-lobes and upon the shell beneath, 

 causing, in some cases, the nacreous layer to be dissolved 

 away. 



In a recent communication to Nature (1920), Orton throws 

 further light upon the habits of the pea-crab. In order to 

 observe what takes place within the mussel a window was 

 cut in one of its shells. The large pea-crabs were usually 



