ECHINODERMA. iy 
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 ot 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 
coelom 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 enterocele, and having the madreporic 
tube connected with it. 
The history of development may be considerably complicated by 
the larvee, which may be of various kinds or degrees of specialized 
modification. 
