DEVELOPMENT OF THE SEA-URCHIN. 121 



tile cilia. When the larva is twenty-three days old the ru- 

 diments of the five tentacles of the sea-urchin appear. By 

 this time the pluteus-form is acquired, and also at this pe- 

 riod the sea-urchin growing upon the deciduous pluteus 

 scaffolding has concealed the shape of the digestive cavity 

 of the larva, and the spines are so large as to conceal the 

 tentacles. The body of the pluteus is gradually absorbed 

 by the growing sea-urchin ; the spines and suckers of the 

 latter increasing in size and number with age, until by the 

 time the larval body has disappeared the young Echinus is 

 more like the adult than the star-fish at the same period in 



Fi^. 84. Hemiaster 1 Phttippii, with the young in two of the marsupia. From 

 Wyville-Thompson's Voyage of the Challenger. 



life. Grube has found that Anochanus sinensis, supposed 

 to have come from the Chinese or East Indian seas, has 

 no metamorphosis ; while Hemiaster cavernosus of Chili 

 was found by Philippi to carry its young in marsupia and to 

 develop directly. 



Several species of sea-urchins in the cooler portions of 

 the South Atlantic, especially at the Falkland Islands and 

 Kerguelen Island,, also develop directly in marsupia or brood- 

 hollows, without passing through a metamorphosis. In Hemi- 



