238 



EVOLUTION AND ANIMAL LIFE 



wormlike creature, jDrovided with cilia or vibratile hairs, by 

 means of which it swims freely about. It changes next into 

 a curious bootjack-shaped body called the pluteus stage. In 

 the pluteus a skeleton of lime is formed, and the final true sea- 

 urchin body begins to appear inside the pluteus, developing 

 and growing by using up the body substance of the pluteus. 

 Starfishes, which are closely related to sea urchins, show a 



similar metamorphosis, 

 except that there is no 

 pluteus stage, the true 

 starfish-shaped body 

 forming within and at 

 the expense of the first 

 larval stage, the ciliated 

 free-swimming stage. 



A young crab just 

 issued from the egg 

 (Fig. 141) is a very 

 different appearing 

 creature from the adult 

 or fully developed crab. 

 The bod}^ of the crab 

 in its first larval stage 

 is composed of a short, 

 globular portion, fur- 

 nished with conspicuous 

 long spines and a rela- 

 tively long, jointed tail. 

 This is called the zoea 

 stage. The zoea changes into a stage called the megalops, 

 which has many characteristics of the adult crab condition, 

 but differs especially from it in the possession of a long, seg- 

 mented tail, and in having the front half of the body longer 

 than wide. The crab in the megalops stage looks very much 

 like a tiny lobster or shrimp. But soon the bod}^ widens, the 

 tail is folded underneath, and the final stage is reached. 



In many families of fishes the changes which take place in 

 the course of the life cycle are almost as great as in the case 

 of the insect or the toad. In the ladyfish (Albida vidpes) the 

 very young are ribbonlike in form, with small heads and very 

 loose texture of the tissues^ the body substance being jelly- 



Fig. 141. — ^Metamorphosis of a crab : a. The zoea 

 stage; b, the megalops; c, the adult. 



