OF ARTS AND SCIENCES. 231 



XVII. 



ON VIVIPAROUS ECHINI FROM THE KERGUELEN 



ISLANDS. 



By Alexander Agassiz. 



Presented, March 8, 1876. 



The function of the deeply sunken petaloid ambulacra of several 

 genera of Spatangoids, such as Moira, Schizaster, Hemiaster, and the 

 like, has thus far remained unknown. Philippi, in 1845, while 

 describing some South American Spatangoids, found in the deeply 

 sunken posterior ambulacra of Hemiaster cavernosus minute Echini, 

 wliich he regarded as the young of the species, though they diflfered 

 widely from the adults, and seemed, from their shape and the nature of 

 their spines, to approach nearer the regular Echini than the Spatan- 

 goids. The Echini of this genus being but rarely found in collections, 

 no opportunity occurred of verifying the observations of Philippi. 

 A somewhat analogous observation was made by Grube, who described 

 more in detail the young of Anochanus (Echinobrissus), which he 

 found living under very similar circumstances, in a cavity opening in 

 the abactinal pole of the specimens. No details of the nature of this 

 cavity having been as yet published, it is not possible to compare these 

 two modes of carrying the young in these two genera more closely. 



In Spatangoids, with deeply sunken ambuhicra, we find, nearly 

 in all cases, that from the sharp edge of the ambulacral groove long 

 spines extend, so as nearly to close the opening of the cavity, entirely 

 bridging it over, and completely concealing from view the ambulacral 

 pores. This arrangement has usually been considered in Spatangoids 

 as a sort of filter to keep foreign particles from affecting the delicate 

 water tubes, which in the Spatangoids perform more or less the func- 

 tion of gills. This is undoubtedly the case in several genera ; but in 

 the case of Hemiaster, and perha^^s in other allied genera, the sunken 

 ambulacral area is used for an entirely different purpose, as was cor- 

 rectly observed by Philippi, that of sheltering the young. 



That the many specimens (eight) found in the two posterior sunken 

 ambulacral areas are really the young of Hemiaster, is of course only 



