DEVELOPMENT. 



143 



FIG. 99. Longitudinal section of three stages in 

 the formation of the enterocoel of Echinus mili- 

 aris (from Korschelt and Heider). 



of sacs, one* of which is called the hydroeoel because it gives 

 rise to the water-vascular system and its lining ; the others 

 constituting the splanehnocoel, because they give rise to the 

 body-cavity and its associated spaces of the adult. The mode 

 of division is very similar 

 throughout the group except 

 in Crinoids, and is described 

 below. 



In Asteroids we may take 

 as type Aster ina gibbosa in 

 which the development has 

 been so fully worked out 

 by Ludwig and later by 

 Mac Bride. The eggs which 

 are 0*5 mm. in diameter 



are attached to stones to which they adhere by means of the 

 vitelline membrane. On about the fourth day the embryo 

 ruptures the vitelline membrane and escapes. It is then found 

 to possess a large preoral lobe (Fig. 100) the edge of which is 

 thickened, constituting the larval organ. The larval organ 

 surrounds a central depression and is covered with specially 

 long cilia, by means of which the larva can swim. Later, in 

 the centre of the concavity of the larval organ, a small eleva- 

 tion is formed. The ectoderm of this contains gland-cells which 

 secrete an adhesive substance by means of which the larva 



fixes itself during the 

 m e t a m o rphosis. The 

 temporary fixation 

 which is possible during 

 larval life appears to be 

 due to a kind of cupping 

 action brought about by 

 the application of the 

 preoral lobe to the sub- 

 stratum and the retract- 

 ion of its central portion. 



FIG. 100. Larvae of Asterina gibbosa (after Ludwig). 

 A, a younger stage, oblique ventral view. B, older 



stage from the side. 

 mouth. 



Lo larval organ; m larval 



and to the sacs which arise from it, the word enterocoel being retained 

 for the peritoneal or body-cavity portions of the latter. We, however, 

 prefer to call the totality of sacs enteroeoelie, distinguishing the water- 

 vascular portion as hydroeoel and the perivisceral portion as splanchnocoel. 

 * Or one pair (see footnote on p. 126). 



