122 THE SEAS 



or are large and strong enough to seek for themselves their 

 natural home on the bottom. 



Worms, starfish, crabs, lobsters, oysters, sea squirts and 

 even the majority of fishes in the sea, spend the first days of 

 their lives drifting about in this manner (see Plate 91). 

 One of the main advantages of this mode of life is that it 

 ensures complete dispersal of the young. Many marine 

 animals live fixed to the rocks and bottom, such as hydroids 

 and mussels, or are at most very sluggish and do not move 

 far afield It is obvious, therefore, that if their offspring 

 were born and hatched " at home " the parental abode 

 would soon become so crowded with young and half-grown 

 children that there would be no room to turn. But if the 

 children are sent out to fend for themselves in the upper 

 water layers they will be carried far and wide by the 

 currents, and those that survive will settle to the bottom 

 many miles from where their parents lived. 



These drifting young of many marine animals are very 

 different in appearance from their parents ; indeed, it is 

 safe to say that if they were shown to a novice it would be 

 impossible for him to say into what they would grow. 

 Because of these extraordinary differences between adults 

 and young, the early stages of many marine animals, when 

 first discovered by naturalists in the plankton catches, were 

 regarded as new species of animals and described and given 

 names of their own. It was only by rearing them, or 

 piecing together successive stages in the life histories from 

 the catches, that the adult into which they were going to 

 develop could be discovered. Who, for instance, without 

 knowing beforehand, would dream of suggesting that the 

 httle animal figured in Plate 43 was the young of the common 

 edible crab ? When first discovered it was not recognized 

 as such and was given the name " Zoea," on this account 

 it is novv known as the zoea stage of the crab. After 



