SEA-UKCHINS. 



175 



Fig. 153. — Brood poucli of HciitucsU:r jthiiipp'ti. coutaiuiug eggs, enlargeii. 



In the sub-f.imily Brissiiia, the genus Hendaster is one of the most intei-esting, on 



necount of the manner in which it carries its young, which develoj) without a metamor- 

 phosis, in receptacles on the apical surface. 



H. philippii, a somewhat heart-shaped species found at Kerguelen Island, has 



certain of the anibula- 



cral plates greatly ex- 



p.nided and de])ressed, 



SI) as to form four deeji, 



thin-walled, oral cups 



^\■hich encroach u]ion 



the cavity of the test, 



.and form niai'sujiia or 



brood-pouches for the 



protection of the 



young. The spines 



are so arranged that a 



kind of covered way 



leads from the ovarial 



opening to the pouch, 



and intliis ]iassagc the 



eggs are kejit in place 



by the .sjiines, two or three of which bend over each egg. In this way they are passed 



to the marsu]iium. The embryos stay within the pouch until the plates of the test have 



rleveloped. Young echini in all stages of development are formed within the pouch, and 



the small ovaries contain so7ne well-developed eggs ready to escape into it as soon as 



there is room. In the young the anal o])ening is nearly central, so that it looks almost 



like a regular urchin. //. cavernoaus is regarded by Agassiz as identical with JI. 



phillip^ni, but H. zonatus and JT. gihbosus are distinct. 



Aeropese rostraUi is remai-kable for its length and narrowness, for its deeply sunken, 



odd anterior ambu- 

 lacrum, and for the 

 eight great sucking 

 discs upon the lat- 

 ter. Aceste beUicK- 

 I '< ra is near Aerope, 

 yet is in appearance 

 I me of the strangest 

 I if sea-urchins, for 

 nearly the whole of 

 its upper surface is 

 occupied by the 

 deeply sunken odd 

 anterior ambula- 

 c r u m, surrounded 

 by a narrow fas- 

 cicle, from within 



which spring large, flattened, paddle-shaped spines. These spines curve over the great 



hollow of the ambulacrum, and underneath them may be seen a number of huge disc- 



Hi 1.'..'. — Upper and under sui-faces of Arf^ti- hi-lHiVn 



