MEM. M. C. Z., IX. No. 2. —ECHINODERMATA. 30 
43. Young Strongylocentrotus, measuring one fifteenth of an inch in diameter, including the spines ; a number 
of long-stemmed pedicellariee have developed on the abactinal side; the tentacles have become quite 
slender, and carry a comparatively large sucking disk ; the spines have lost their embryonic character, and 
have assumed the general appearance of those of the adult. The anal system is very prominent, a large 
circular opening covered by a single plate, a, leaving the anal opening on one edge of the anal system. 
44. The test of a young Sea-urchin, in the stage of figs. 41, 42, stripped of its spines, seen from the actinal side ; 
the actinal system is comparatively large, and, as in Cidaris, the tubercles are large and few in number ; no 
miliary tubereules are as yet formed. The teeth are simple, and the accessory parts of the jaws are not 
developed. 
