189 



size of four and a quarter inches until the fourth year at 

 the earliest. 



There is abundant evidence to show that the adult 

 male and female crabs do not cast every year, but 

 probably only once in two years. There appears to be 

 little doubt, moreover, that in the older crabs ecdysis may 

 take place less frequently than once in two years, and 

 Williamson has given several instances of crabs in which 

 the shell is undoubtedly more than two years old. In the 

 Nordsee Museum, Heligoland, there is a female Cancer, 

 with a carapace breadth of 17 cm., having attached to the 

 shell, an Anomia 5'8 cm. in width. 



Although there appears to be a stage in the life- 

 history of Cancer after which ecdysis is only triennial, no 

 attempt has been made to determine at -which period the 

 change from a biennial to a triennial growth takes 

 place. 



Such an investigation presents innumerable 

 difficulties, and the only way in which the problem can be 

 satisfactorily solved is either by having large crabs under 

 observation in captivity for many years, or by the careful 

 analysis of a great number of exact measurements made 

 for the purpose. Neither of these methods has been 

 followed, and although Williamson has been able to 

 gather a mass of extremely useful information, his 

 statistics do not appear to be of much value in the 

 elucidation of this particular problem. 



Our present knowledge with regard to the frequency 

 of casting may be briefly summarised as follows. In the 

 early stages the young crab casts frequently. As it grows 

 older the periods between successive castings become 

 longer, and it is probable that after reaching maturity 

 both males and females cast only once in two years. 

 There is scattered evidence to show that many large crabs 



