EVOLUTION OF RADIAL SYMMETRY 



621 



ence takes place, which points clearly to the splitting of the 

 Echinoderm stem into two main branches, corresponding with 

 two different sets of habits. In the Eleutherozoan stock, 

 represented by the development of the Asteroidea, the disc 

 became ilexed ventrally on the stalk, so that the mouth and 

 podia were brought within reach of material drifting along the 

 bottom, which the podia were employed to seize. As a con- 

 sequence the base of the stalk was brought near the mouth, and 

 so it came about that the hydrocoel, when it became a ring, 

 encircled both. In the Pelmatozoan stock, on the other hand, 

 the podia and mouth are rotated upwards and backwards from the 



Fia 296 — Figures to show the supposed connexion of Elentherozoa and Pelmatozoa. A, 

 common fixed ancestor of the two stocks, still bilaterally symmetrical ; B, primitive 

 Pelmatozoon ; C, primitive Eleutherozoon. a, Anus ; a.c, anterior coeloni ; l.p.c, 

 l.p.c^, left posterior coelom ; m, mouth ; p, primary pore-canal ; r.p.c, right posterior 

 coeloia. 



stalk, which thus came to have an aboral position (Fig. 296, B). 

 The podia are thus placed in a favourable position for capturing 

 free-swimming organisms, which their cilia sweep toward them. 

 It is worthy of note that a similar change of position of the 

 mouth occurs in other groups of animals which have similar 

 habits (Polyzoa Entoprocta, Tunicata). 



The division therefore of the phylum must have occurred at 

 an extremely remote epoch, before the hydrocoel was a closed 

 ring, and before, therefore, radial symmetry was completely 

 attained. 



Turning now to the question of the origin of the classes of 

 Elentherozoa, we find that the study of development strongly 

 reinforces the views gained from the study of adult anatomy. 

 The Asteroidea are the most primitive group ; only in their case 



