XVI 



ECHINODEKMATA 



563 



radial symmetry, and was encouraged. But when this stage had 

 been reached, the study of ontogeny seems to indicate that the 

 Echinoderm stock became split into two stems. For in Crinoid 

 development the stomodaeum, into which the tentacles project, becomes 

 rotated backwards, so that it eventually occupies the posterior pole 



r.pc 



FIG. 414. Diagrammatic reconstruc- 

 tion of the common ancestor of 

 the phylum Echinodermata, and 

 diagrams show the modifications 

 which the descendants of this 

 ancestor underwent in becoming 

 the ancestors of the Eleutherozoon 

 and Pelmatozoon stocks.\ 



A, Dipleurula ancestor. B, ancestor of 

 the Eleutherozoa. C, ancestor of the Pel- 

 matozoa. o, anus ; o.c, anterior coelom ; 

 l.hy, left hydrocoele; l.p.c, left posterior 

 coelom; m.p, madreporic pore; r.hy, right 

 hydrocoele ; r.p.c, right posterior coelom. 



of the body and is directed upwards ; but in Asteroid development the 

 mouth is moved to the left, and the disc is then flexed on the stalk in 

 such a way that the mouth looks downwards. 



As a consequence of this flexure, when the hydrocoele ring 

 becomes completed by the meeting of its two ends, it encircles 

 the base of the stalk in Asteroidea, but not in Crinoidea, where it is 

 at the opposite end of the body from that occupied by the stalk. 



a 



