41 



radially, or as a series of pouches around the front part of 

 the alimentary canal, so as to produce the first traces of the 

 more or less complete radial symmetry which is such a 

 marked feature in all Echinoderms. If the nervous system 

 was still in its primitive difi'used condition, it may readily 

 be imagined that the formation of radially arranged regions 

 of the coelom, which were being evolved into vessels with 

 tentacle-like projections to the exterior, would result in the 

 concentration of the nervous tissue along these lines ; and 

 if, as is probable, there was previously a nervous concentra- 

 tion around the mouth, then the newly formed nerve bands 

 would naturally radiate outwards from the circum-oral ring 

 (see fig. 16). The other characters which the common 



Fig. 16. Hypothetical ancestor of the Echinodermata. m, mouth; a, anus; 

 n, nerve cord; r, water- vascular diverticulum from archenteron. 



ancestor of the Echinodermata must have acquired are : — a 

 tendency towards the formation of the water-vascular caeca 

 in fives, and as a result the pentagonal symmetry of most of 

 the systems of the body ; the development of a large ccelom 

 or body cavity; and the deposition of calcareous matter in 

 the deeper parts of the integument. 



The ancestral line then probably split into two series : — 

 the one being continued into the progenitors of the 

 Asteroids, the Ophiuroids, and the Crinoids, and the other 

 being evolved into the primitive Echinoids and Holothurians. 



