104 



CONNECTICUT GEOL. AND NAT. HIST. SURVEY. 



[Bull. 



by its further growth, and the development of plates and spines, 

 it is able to assume the habits of the adult. 



Fig. 18. Arbacia punctulata. Oral 

 surface of young urchin shortly after 

 the metamorphosis. (After Brooks.) 



The clypeastroids pass through stages very similar to those 

 of the regular sea-urchins. The sand-dollar, for example, de- 

 velops from a long-armed pluteus, which settles to the sand at the 

 sea bottom and gives rise to the young sand-dollar. This has at 

 first a few relatively long spines and large tube-feet. There is 

 now a nearly complete radial symmetry, the intestinal opening 

 at this stage being near the center of the upper surface, some- 

 what as in the regular urchins. With the further growth of the 

 animal, its bilateral symmetry becomes more marked, and the 

 anus is gradually shifted to its final position near or beneath the 

 edge of the disk. 



Grave* has been successful in rearing large numbers of the 

 key-hole urchin from the fertilized eggs, and following their 

 transformations to the adult form. The egg of this species 



C. Grave. Biol. Bulletin, vol. v, 1903. 



