Chap. III.] 



ECHINODER MS. 



59 



explicable. To understand it we must make use of 

 the method of comparison, and appeal to palaeonto- 

 logical evidence. When \ve do this we find that the 

 oldest forms were, like the still extant Pentacrinus, 



Fig. 2L. Astropecten irregularis. 

 m, Mndreporite. 



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 cnelornate Metazoa a 

 hard supporting structure to which we can apply the 



