STAR-FISH AND SEA-UKCHINS. 2C7 



left for its feet to come through, we should have a 

 mental picture of an Echinus. It would only be 

 necessary to add the curious apparatus of teeth 

 (Figs. 40 and 42), which occurs in the Echinus, to 

 increase the size of tlie spines and pedicellariae, and 

 to make a few other such minor alterations ; but in 

 all its main features an Echinus is merely a Star- 

 fish with its five rays calcified and soldered together 

 so as to constitute a riicid box. 



This echinoid type itself varies considerably 

 among its numerous constituent species as to size, 

 shape, length and thickness of the spines, etc. ; but 



Fig. -12.— Teeth of Kcliiiiu^- (Irom CisseU's " Nat. Hist.") 



I need not wait to go into these details. Again, 

 merely inviting momentary attention to the develop- 

 mental history of these animals, I may remark that 

 the phases of development through which an indi- 

 vidual Echinoderm passes are not less varied and 

 remarkable than are the permanent forms eventually 

 assumed by the sundry species. 



Katura I Movem en fs. 



Turning now to the physiology of the Star-fish 

 group, I shall begin by describing the natural 

 movements of the animals. 



