Chap. TIL] ECHINODERMS. 59 



explicable. To understand it we must make use of 

 the method of comparison, and appeal to palaeonto- 

 logical evidence. When we do this we find that the 

 oldest forms were, like the still extant Pentacrinus, 



Fig. 21. Astropecten irregularis. 

 7w,Madreporite. 



fixed on a stalk (Fig. 22) ; in other words, the ances- 

 tors of the Crinoids being fixed forms had to develop 

 their organs in different directions around a common 

 centre, so that, from whatever point prey or enemy 

 approached them, they would be prepared for and 

 ready to meet them. 



In the great majority of this group we observe 

 for the first time among the coelomate Metazoa a 

 hard supporting structure to which we can apply the 



