74 THE SEAS 



when they begin to assume the adult form. The young of 

 animals which live in very deep water would have difficulty 

 in rising several miles to the surface for a floating existence 

 of perhaps only a few weeks or even days, and then dropping 

 back that great distance, and as a result we find that the 

 development of deep-sea animals is " direct," i.e., there is 

 no strange " larval " stage, when the young are free in 

 the sea, between the egg and the fixed adult, but the 

 latter develops directly from the egg. The advantage of 

 the former method in ensuring the wide distribution of 

 the animal concerned will be obvious. The spawning 

 habits of bottom animals are many and various, but, 

 as with the shore animals, can be divided into three 

 main types, where the reproductive products are shed 

 indiscriminately, where protected spawn is laid, and 

 where the developing young are carried about by the 

 parent. 



Bottom animals do not make long migrations like the 

 fish. This is impossible in the case of the many attached or 

 rooted animals while a large number of the others are too 

 sluggish to move far. Octopuses certainly move from place 

 to place causing, as we saw, great damage when they 

 suddenly invade new areas, but this is probably the result of 

 exceptional temperature conditions and not a seasonal 

 occurrence. Crustaceans, at any rate the larger ones, are 

 active beasts and can move about freely ; many, like the 

 edible crab, coming inshore in the summer and then retiring 

 into deeper water in the winter. In the summer the shore 

 waters are warmer than the deeper water, but in the winter 

 the reverse is true. Another animal which can move 

 about is the scallop, especially the smaller " queen " 

 {Pecten opercularis) which, by the continuous flapping of its 

 shell valves, is able to swim about, hinge side hindmost 

 (a process which is reversed when the animal is alarmed) 



