ECHINODERMA.. 



11 



The ciliated larva appears, in all Echinoderms, to be formed as a 

 hollow sphere (blastosphere) bounded by a single layer of cells ; it 

 gives rise to a two-layered sac (Gastrula) by ingrowth at one point 

 or invagination. As it elongates, one end becomes anterior and one 

 posterior ; one side ventral and one dorsal. 



The central cavity formed by invagination, the archenteron, gives 

 rise to two outgrowths which, as they will form the general ccelom 

 or body-cavity, may be called the right and left enterocceles ; these 

 increase in size, the left growing larger than the right: later on, 

 part of the wall between them breaks down and a continuous body- 

 cavity is formed, while another part of the wall of each sac unites 

 to make a supporting mesentery. From the surface of the body an 

 ingrowth leads to the formation of a tube which connects the 

 ccelom with the exterior ; while this is forming, there appears, in 

 various ways in various Echinoderms, a system of water-vessels 

 developed from part of the enterocoele, and having the madreporic 

 tube connected with it. 



The history of development may be considerably complicated by 

 the larva?, which may be of various kinds or degrees of specialized 

 modification 



