122 



ZOOLOGY. 



aster Philippii Gray (Figs. 84 and 85), from the latter island, 

 certain of the ambulacral 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 marsupinm, 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. Marpupium of Hemiaster Philippii, containing egg?. Much magnified 

 From Wyville-ThoiniJsoii'e Voyage of the Challenger. 



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

 when they leave the marsupium they are about two and a 

 half millimetres long. In Cidaris nutrix Wy ville-Tliompson 

 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 larva? (pluteus, etc.) were seen by the Challenger 

 Expedition in the Southern Ocean. 



