XVI 



ECHINODERMATA 



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 stomodaeiim, into which the tentacles project, becomes 

 rotated backwards, so that it eventually occupies the posterior pole 



r.pc 



mp 



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, Diplmi'uhi aiipi'.stor. B, niici-stm -cif 

 the Eleuthmi/.uii. C, unrrslur nf Un- Pel- 



niuto/oa. a, nuns ; n.c, antiM im- i lorn; 



I. hi/, left liyilnirnrlc; /./>., left. postninr 

 coelom ; /./>, iim'liriioi'ir purr; c./i.i/, ri^ht 

 ; r.p.c, ri.^lit. posterior coelom. 



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

 moutli is moved to the left, and the disc is then flexed on the stalls in 

 such a wai/ that the mouth looks downwanh. 



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 



