204 



ZOOLOGY. 



aster PJnlippii Gray (Figs. 147, 148), from the latter island, 

 certain of the ambulacra! plates are greatly expanded and 

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

 sinking into and encroaching upon the cavity of the test, 

 and forming very efficient protective marsupia. " The 

 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. 148. -Marsupium of Hemiaster Philippii, con raining eggs. Much magnified. - 

 From Wyville-Thompsou's Voyage of the Challenger. 



plates once begin to develop, rapidly assume the parent form ;. 

 Avhen they leave the marsupium 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 Northern At- 

 lantic are either known or supposed to be metabolous ; and 

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

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

 Expedition in the Southern Ocean. 



