232 



PROCEEDINGS OF THE AMERICAN ACADEMY 



probable, from the fact that the genital openings, which are unusually 

 large, open directly into the upper part of their sunken area ; so that the 

 eggs (or more probably an imperfectly developed Pluteus, like that of 

 Echinaster) on escaping from the genital openings would readily find 

 their way into the artificial cavity formed by the spines which conceal 

 the presence of the sunken areas. 



Unlike many Echini, the ovaries of this genus are small, consist- 

 ing of compact grape-like clusters of eggs, in very different stages of 

 development, a few of the eggs only attaining a considerable size 

 (nearly 1 mm.) and apparently ready to escape into the sunken area, as 

 soon as the place should be left unoccupied by the preceding brood. 

 No two of the small Echini were in the same stage of development : 

 they varied in size from 2 mm. to 3 mm., the smaller specimens having 

 a somewhat pentagonal outline, with rounded angles ; the larger ones 

 were more nearly elliptical and cylindrical in shape. In the smaller 



Fig. 2. 



specimens (Fig. 1), the spines were short, straight; the longest, and 

 only a few in each interambulacral area, about one-fifth the length 

 of the axis, while the greater number were mere tubercles, scarcely 

 rising above the level of the test. In the largest specimens (Fig. 2), 



Fig. 1. Young Hemiaster, measuring 2 mm., seen from the abactinal pole. a,a, 

 ambulacral spaces. The peripetalous fasciole is already developed. 



nmciMrVint nldpr Hpmiastpr nipnsiirincr S mm. appn frnm 



ral spaces, i lie peripetalous lasciole is alreaay develop 

 Somewhat older Hemiaster, measuring 3 mm., seen from the ac 

 side, a, a, ambulacral areas. 



tinal 



