XVI 



ECHINODEEMATA 



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 Grinoid 

 development the stomodaeum, into which the tentacles project, iecomes 

 rotated iackwards, so that it eventually occupies the posterior pole 



FlH. 414. — Diagrammatic reconstruc- 

 tion ot the common ancestor ot 

 tlie phylum Echinodermata, and 

 diagrams show tlie moditications 

 which the descendants of this 

 ancestor underwent in becoming 

 tlie ancestors of the Eleutherozoon 

 and Pelmatozoou stocks. 



A, Dipleurula ancestor. B, ancestor of 

 tlie Eleutherozoa. 0, ancestor of the Pel- 

 matozoa. a, anus ; ti.c, anterior coelom ; 

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

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

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



of 'the hody and is directed upwards ; but in Asteroid development the 

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

 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 Crmoidea, where it is 

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



