SEA-FISHERIES LABORATORY; 471 



this regular migration is the influence of temperature. 

 In the winter the deeper layers of the offshore waters are 

 warmer than the inshore waters, and hence the former 

 are most suitable for the spawning time. In the summer 

 the shallower inshore waters become much warmer, and 

 here the young larvae are hatched. It is also reasonable to 

 suppose that the warm water of the inshore regions is 

 specially favourable for the somewhat critical period of 

 casting. The necessity for food may also have an 

 important bearing on the yearly migration. 



I give below a summary of the life-history of an 

 adult female crab between two successive processes of 

 ecdysis. The condition of the gonads at the various stages 

 is discussed. In the example given, it is assumed that 

 the crab did not spawn in the winter following fertilisa- 

 tion. The period elapsing between fertilisation and the 

 first spawning appears to depend altogether on the 

 condition of the gonads. As a rule, the ovary is very 

 small and pale when ecdysis takes place, and in such a 

 circumstance the eggs cannot possibly be ready for 

 spawning in the course of three or four months. 

 Consequently the eggs are not shed until the second 

 winter, i.e. fourteen months after fertilisation. In some 

 crabs the ovary is fairly ripe when ecdysis takes 

 place, and in such cases the eggs will probably be 

 extruded in a few months. 

 First Year. September — The crab casts and is fertilised. 



It then migrates to offshore waters for the winter. 

 It returns to inshore waters in the spring of the second 

 year. In the following autumn offshore migration again 

 takes place. 



Condition of ovary. At first it is poorly 

 developed and pale in colour, and the eggs are without 

 food-volk. The ovary gradually ripens, first becoming 



