EVOLUTION OF RADIAL SYMMETRY 



621 



eiice takes place, which points clearly to the s})litting of the 

 Ec'lunoderiu stem into two niaiu branches, corresponding with 

 two different sets of habits. In the Elentlierozoan stock, 

 represented by the development of the Asteroidea, tlie disc 

 l)ecame ilexed ventrally on the stalk, so that the moutli and 

 podia were brought witliin reach of material drifting along tlie 

 l)ottoni, 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 tlie hydrocoel, when it became a ring, 

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

 tlie podia and mouth are rotated upwards and backwards from the 



Via. 296 — Figures to sliow tlu^ Mijjpo.sed connexion of Eleutlierozoa ami Pelniatozoa. A. 

 coiunion fixed uncestoi- of tlie two stocks, still bilaterally symmetrical ; B, primitive 

 relmatozooii ; C, primitive Eleutherozooii. a. Anus ; a.c, anterior coelom ; l.p.c, 

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

 coelom. 



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

 The podia are thus placed in a favourable position for capturing 

 fiee-swimniing organisms, which their cilia sweep toward them. 

 It is wortliy of note that a similar cliange of position of the 

 mouth occurs in other groups of animals which liave 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 tlie question of the origin of tlie clas.ses of 

 Eleutlierozoa, we find that the study of development strongly 

 reinforces the views gained from the study of adult anatomy. 

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



