204 ZOOLOGY. 
aster Philippit Gray (Figs. 147, 148), 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 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 Philippit, containing eggs. Much magnified.— 
From Wyville-Thompsou’s 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 millimetreslong. In Cidaris nutriz 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 Hchi- 
noderm larve (pluteus, etc.) were seen by the Challenger 
Expedition in the Southern Ocean. 
