123 



ZOOLOGY. 



aster Philijopii Gray (Fig-. 84 and 85), from the latter island^ 

 certain of tile ambulacral plates are greatly expanded and' 

 depressed " so as to form four deep, tliin-walled oral cups, 

 sinking into and encroaching upon the cavity of the test, 

 and forming very efficient protective marsupia." Th& 

 spines are so arranged that a kind of covered passage leads 

 from the ovarial opening into the marsupium, and along 

 this passage the eggs, which are very large (a millimetre in 

 diameter) are passed and arranged in rows, each egg being: 

 kept in place by two or three spines bending over it. Here 

 the eggs develop, and the embryos, after the calcareous- 



Fig. 85. — Marsupinm of Hemiaster PhUippii, coutaining egg?. 

 Prom WjTille-Tliomp=uu"s Voyage of tlie Challeuger. 



Much magnified.- 



plates once begin to develop, rapidly assume the parent form •, 

 when they leave the marsupi um they are about two and a 

 half millimetres long. In Cidaris nutrix Wyville-Thompson 

 the eggs are protected in a sort of tent by certain spines 

 near the mouth. Here the young develop without a meta- 

 morphosis. The allies of these forms in the Xorthern At- 

 lantic are either known or supposed to be metabolous ; and 

 Sir Wyville-Thompson states that no free-swimming Eehi- 

 noderm larvae (plutens, etc.) were seen by the Challenger 

 Expedition in the Southern Ocean. 



