128 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



The larva soon becomes asymmetrical, as shown in 

 ventral view in Fig. 74, and the pre-oral arms («') begin 

 to disappear, while the dorsal, lateral arms {q) become 

 longer than any of the others. 



A large circular opening makes its appearance on the 

 right side of the body (ventral view) , between the bases 

 of the post-oral arm (c) and the dorsal, lateral arm {q) and 

 through this opening the ambulacral feet (a 6) of the sea- 

 urchin may now be protruded. They are five in number, 

 one for each ray of the sea-urchin, and around them there 

 is a circle of fifteen flattened, perforated plates, the first 

 set of spines of the young sea-urchin. 



The stomach (s) is now pushed over on to the left side 

 of the body, and a granular belt (ech) with pigment 

 spots, around its right side, indicates the position of the 

 developing body-wall of the ab-oral surface of the sea- 

 urchin. The pluteus now becomes still more distorted, and 

 in about twenty-four hours it assumes the form shown in 

 Fig. 75, which is a dorsal view. The mouth and pre- 

 oral arms («', a") of the pluteus are pushed to the left, 

 and the growing sea-urchin now fills nearly the whole 

 body. The two ventral, lateral arms (n, n) are nearly 

 parallel to each other, and the post-oral arms (c) and dor- 

 sal, lateral arms {q) are thrown back towards, the posterior 

 end of the body. The five ambulacral feet are now pro- 

 truded from the surface of the body, and a disc of small, 

 calcareous plates appears in the sucker with which each 

 of them ends. Between their bases are the fifteen spines, 

 (s), arranged in five sets, of three each. 



For some time the larva is able to bend back the arms 

 as shown in the figure, and, protruding its feet, to crawl 

 as an echinoderm ; or, pulling back the feet, and pushing 

 the arms into their original position, to swim as a pluteus. 



