EMBRYOLOGY OF ECHINODERMS. 



125 



an opening, winch at this period of the animal's life serves a 

 double purpose, that of mouth and anus also, for at this opening 

 it both takes in and rejects its food. We shall see that before 

 long a true mouth is formed, after which this first aperture takes 

 its place opposite the mouth, retaining only the function of the 

 anus. Presently from the upper bulging extremity of the diges- 



Fig. 158. Fig. 159. Fig. 1fiO. Fig. 161. 



m a 



tive cavity, two lappets, or little pouches, project (t0w',Mg. 161) ; 

 they shortly become completely separated from it, and form two 

 distinct hollow cavities (iviv 1 , Fig. 162). Here begins the true 

 history of the young Star-fish, for these two cavities will develop 

 into two water-tubes, on one of which the back of the Star-fish, 

 that is, its upper surface, covered with spines, .will be developed, 



Fig. 163. 



while on the other, the lower surface, with the suckers and tenta- 

 cles, will arise. At a very early stage one of these water-tubes 

 (w', Fig. 163) connects with a smaller tube opening outwards, 

 which is hereafter to be the madreporic body (6, Fig. 163). 

 Almost until the end of its growth, these two surfaces, as we 



Fig. 15S. Larva just hatched from egg ; a thickened pole. 



Fig. 159. Larva somewhat older than Fig. 158 ; m a depression at thickened pole. 



Fig. 160. Larva where the depression has become a digestive cavity d, opening at a. 



Fig. 161. Earlets, w w' (water-tubes), developed at the extremity of the digestive cavity d ; m mouth. 



Fig. 162. More advanced larva ; a d c digestive system, i> vibratile chord, m mouth. 



Fig. 163. Profile view of larva ; b madreporic opening, w' earlet, a d digestive system, m mouth, 

 )' vibratile chord. 



Fig. 164. Larva showing mode of formation of mouth m, by bending of digestive cavity o 



