4 rRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



liemisomes, but among those species which occupy 

 helicoid univalve shells, marked asymmetry has 

 developed. The majority of the Lamellibranchs and the 

 Gastropods possess shells which have acted as a shield 

 or protection to the soft tissues, but which re-acting 

 upon them have produced diversity between the two 

 hemisomes. The modification appears to be very ancient. 

 Among- the highest class of Molluscs, the Cephalopoda, 

 remarkable asymmetry appears in. the male, in hectoco- 

 tvlisation, a modification of one of the arms to contain the 

 generative products. The antiquity of hectoootjdisation in 

 geological time appears to be very great. Passing on to 

 Crustaceans we find in Pagurids, the hermit crabs, a 

 moulding of the body into non-symmetrical form to fit 

 the helicoid shell of a Mollusc, a consequent shrinking 

 of the chela adjacent to the shell and an increased 

 growth of that more remote from it. The case is very 

 similar to that of the mollusc. 



Among the Crustaceans there are also certain crabs, 

 such as Gelasimus pugilator, in which, in the male, one 

 claw is a great deal larger than the other; according to 

 Yarkes, of Harvard, the right claw is the larger in 52 

 per cent, of examples, while in 48 per cent, the augmenta- 

 tion is in the left; he finds that the right clawed or right 

 handed crabs are larger than the left handed ones, and 

 less variable also. The same phenomenon is seen in 

 Gelasimus maracoani and in G. voeans, the calling crab 

 (so named from its habit of beckoning with the big claw), 

 in Cardisoma and many others; among the prawns also 

 as in Axius stirhynchus and Callianassa suhterranea. The 

 possession of one large and powerful claw may make the 

 male animal more formidable than would the possession 

 of iwo of more moderate size. As to the antiquity of this 

 acquired peculiarity I have no information, 



