THE MOVEMENTS OF SHORE ANIMALS 175 



for food. The shallow inshore waters are, of course, 

 warmest in the summer months, and it is here that the eggs 

 hatch and the larvae develop. On the other hand, in winter 

 the deeper layers of the offshore waters are distinctly warmer 

 than the shallow waters inshore, and it is possible that it 

 is on this account that they are sought in winter for the 

 act of spawning. This cannot, however, be the complete 

 explanation for, as Williamson points out, by the time the 

 outward migration begins the inshore waters are still the 

 warmest. Neither Williamson nor Pearson seems to have 

 considered whether a possible explanation might not be 

 the need for seeking more tranquil waters when the autumn 

 storms come on. 



Similar movements are undertaken by lobsters and 

 prawns, and in the same way the fish of rock pools frequent 

 deeper water in winter. 



Of great interest are the breeding migrations of Crustacea 

 which are in course of becoming terrestrial. Crabs of 

 the genus Geocarcinm, as stated in Chap. XH, live in 

 sheltered situations several miles from the sea, and make 

 their way in spring in herds to the sea-shore in order to breed. 



Eliot (1910) remarks that both adult and young Nudi- 

 branchs appear and disappear suddenly in considerable 

 numbers, particularly before spawning and after hatching. 

 Notwithstanding that Alder and Hancock were induced to 

 disbelieve in these migrations, Eliot considers there is a 

 good deal of evidence for their occurrence. The method 

 by which the migration is effected is not known, but as 

 creeping is an extremely slow process, it is suggested that 

 the animals make use of currents, in which they suspend 

 themselves by means of slime threads. 



Vertical Movements of Burrowing Forms in Relation to 

 the Tides. — A consideration of the rhythmical movements of 

 shore animals in relation to the tides is deferred till Chap. XI. 

 It is not out of place here, however, to call attention to 

 the vertical movements which are regularly performed by 

 burrowing worms and molluscs, as the tide ebbs and flows. 



When the water has left the sand, worms and bivalves 



