278 



SKXCAL DIMORPHISM 



mentioned above, the exertion of these organs must in the 

 male be frequent and energetic. Thus, all we know of the 

 history of this species is in accordance with the view that 

 the sexual dimorphism and the remarkable metamorphosis 

 are directly due to functionally produced modifications, and 

 not to independent variations. The original cause of the 

 divergence from the normal Isopod structure was doubtless 

 the acquisition of the habit of blood-sucking, which led 

 gradually to the postponement of the generative function to 

 the last stage of life, and secondarily, in consequence of the 

 highly nourished condition of the mother, to the viviparous 

 mode of reproduction. 



In the parasitic Isopoda forming the family Bopyridse 

 sexual dimorphism is very strongly marked. In the genus 

 Bojpyrus the female is five or six times as large as the male, 

 much depressed in form of body and also asymmetrical. 

 The animals live in a stationary condition in the branchial 

 cavity of prawns, Bopyrus squillarum, a common British species, 

 being parasitic upon the common prawn, Palcemon serratus. 

 The shape of the body is evidently moulded by the shape of 

 the cavity which the animal inhabits, and the asymmetry is 

 due to the permanent bending of the axis of the body. The 

 appendages of the mouth are rudimentary, consisting of a 

 pair of oval flat maxillipeds, and small pointed mandibles. 

 The seven pairs of legs are very short, but strong, 

 and furnished each with a strong claw by which the 

 parasite holds on to its host. Each leg is furnished with 

 large incubatory plates beneath which the young are 

 fostered. The abdomen, or pleon, is narrower and shorter 

 than the pereion, and each of its segments has a pair of 

 appendages in the form of transverse flat plates. The 

 minute male is found fixed among these plates on the ventral 

 side of the female, and not being too large for the space 

 available remains symmetrical in form, though much degen- 



