THE LIFE CYCLE 



97 



developing and growing by using up the body substance of 

 trie pluteus. Star-fishes, which are closely related to sea- 

 urchins, show a simi- 

 lar metamorphosis, 

 except that there is 

 no pluteus stage, the 

 true star-fish-shaped 

 body forming, with- 

 in and at the expense 

 of the first larval 

 stage, the ciliated 

 free-swimming stage. 



A young crab just 

 issued from the egg 

 (Fig. 48) is a very 

 different appearing 

 creature from the 

 adult or fully devel- 

 oped crab. The body 

 of the crab in its 

 first larval stage is 

 composed of a short, 

 globular portion, fur- 

 nished with conspicuous long spines and a relatively 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 espe- 

 cially from it in the possession of a long, segmented 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. The tail soon disappears and the 

 body widens, 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 lady-fish (Albula 

 vulpes) the very young (Fig. 49) are ribbon-like in form, 



FIG. 



48. Metamorphosis of the crab, a, the zoea 

 stage ; 6, the megalops ; c, the adult. 



