SEA-URCHINS. 



175 



Fid. 153. —Brood pouch of Haniaster pkilippii containing eggs, enlarged. 



In the sub-family Brissina, the genus Hemiaster is one of the most interesting, on 

 account of the manner in which it carries its young, which develop without a metamor- 

 phosis, in receptacles on the apical surface. 



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

 certain of the ambula- 

 cral plates greatly ex- 

 panded and depressed, 

 so as to form four deep, 

 thin-walled, oral cups 

 which encroach upon 

 the cavity of the test, 

 and form marsupia 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 in this passage the 

 eggs are kept in place 

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

 to the marsupium. The embryos stay within the pouch until the plates of the test have 

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

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

 there is room. In the young the anal opening is nearly central, so that it looks almost 

 like a regular urchin. H. cavernosus is regarded by Agassiz as identical with H. 

 phiUippii, but S. zonatiis and H. gibhosus are distinct. 



Aeropese rostrata is remarkable 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 bellidi- 

 _^^^ .,__„-, /era is near ^ero»e, 



> .> v^WjwtWK^?*' JTOar? ^"^^^ VnjfeA ''-.^frSr'fr yet IS m appearance 



one of the strangest 

 of sea-urchins, for 

 nearly the whole of 

 its upper surface is 

 occupied by the 

 deeply sunken odd 

 anterior ambula- 

 crum, surrounded 

 by a narrow fas- 

 ciole, 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- 



FiGS. 154 and 155. — Upper and under surfaces of Aceste hellidifera. 



