DRIFTING LIFE 123 



hatching, the young crab remains in this stage for some 

 time during which it undergoes several moults. Finally, it 

 suddenly moults into a stage quite unlike the zoea and more 

 like the adult crab ; albeit when first found it was not 

 recognized as such and was labelled " megalopa " (Plate 44). 

 From this stage it moults into a perfect little crab. So we 

 see that in the life-history of one animal, the crab, the 

 successive stages are so different that no less than three 

 animals have been thought to exist, the zoea, the megalopa 

 and the crab, which were in reality all crab. 



These animals which appear for only a short period in the 

 drifting community form the temporary members of the 

 plankton, and their appearance in the upper water layers 

 depends, of course, on the times at which the parents 

 spawn, and this gives rise to the seasonal changes in the 

 plankton mentioned on page 244. It is chiefly in the coastal 

 regions that these temporary drifters are found because 

 of the great wealth of bottom life in the shallower regions. 



Distribution of Plankton 



Having given the reader some idea of what composes this 

 drifting life or plankton, let us now examine what is known 

 of the distribution of the plants and animals that form it in 

 the sea. 



To begin with it must be realized that besides the immense 

 horizontal area of all the oceans and seas of the world, all 

 of which are inhabitable, the waters also have a considerable 

 depth, the average of which is as rnuch as two miles. 

 Seeing then that organisms that live in the water layers 

 themselves are free to move up and down, as well as in a 

 horizontal direction, it is evident that we have two types 

 of distribution to deal with, namely, horizontal or geo- 

 graphical distribution, and vertical or depth distribution. 



Much work has been done on the geographical distribution 



