Fourth Section. 



Echinoderma. 



General Review. 



§ 157. 



The Echiuoderma form a strictly -defined and iudepeudeiit 

 group, owing to the well-marked development of a special type. 

 It is distinguished from the Coelenterata by the differentiation 

 of the enteric canal and formation of a perienteric cavity (coelom). 

 The calcification of the integumentary layer (perisome)^ which 

 encloses this body-cavity, and the radiate arrangement of the 

 body, which consists of more than two antimeres, affords a fairly 

 sharp boundary between it and the higher divisions. This 

 difference between the fully-developed form of the Echinoderma 

 and that of the other types does not obtain in the larval stages, 

 so that in these we can make out genetic relations with the other 

 types. These are the more to be insisted on, as the actinoid type 

 of the Echinoderma was regarded as a reason for uniting them and 

 the Coelenterata into a large group— that of the Eadiata ; a union 

 which on close examination cannot be justified. Such a union is 

 inconsistent with the close relationship between them and the 

 Vermes, especially the Anuelides and Gephyrea. The observation 

 that the internal as well as the external organisation of the Echino- 

 derma gives evidence of metamerism, has further justified their 

 separation from the Coelenterata, and has given rise to the hypo- 

 thesis propounded by Hackel, which regards the Echinoderma as 

 derived from colonies of worm-like organisms. 



The larval form of the Echinoderma agrees exactly with the larva 

 of the Vermes. Here, as in many of the latter, a new organism is built 

 up within the body of the larva. But radial gemmation takes place, 

 and a number of individuals are produced, and thereupon the pliEeno- 

 menon enters upon the well-known series of changes. Gradually the 



