BEHAVIOUR 



295 



is crawling — that is, more or less transversely to their arms. A 

 starfish which falls on its back can right itself. In this process 

 also one or two arms take the lead, turning over so as to touch the 

 ground and holding on with their tube-feet, while the other arms, 

 probably stimulated by the first movers, arch over by muscular 

 action till the creature 

 topples over on to its oral 

 side. The arms which take 

 the lead are those which 

 had previously led in 

 crawling. Pedicellariae 

 bend towards the site 

 of a gentle stimulus on 



an 



Fig, 220. — The bipin- 

 naria larva of a star- 

 fish, in ventral view. 



an. 



Anus ; b., 

 band ; b'. 

 m., mouth. 



postoral ciliated 

 preoral band ; 



The depressed region between the 

 ciliated bands is shaded. 



Fig. 221. — The larva of a starfish at the fixation 

 stage, viewed from the right-hand side. — 

 From MacBride, after Johannes Miiller. 



The bipinnaria larva has passed into a stage known as the 

 brachiolaria by the development of three fixing processes 

 at the anterior end. Ast., rudiment of the future body of 

 the starfish ; b., postoral ciliated band ; h' , preoral band ; 

 fix., fi.xation processes at the anterior end of the larva. 



The larva is seen from the right-hand side, and its position is 

 inverted from that of Fig. 220. 



the skin, opening their blades and closing them upon any object 

 that comes against their inner sides. The larger kind are set in 

 motion by a weaker stimulus than is needed to move the smaller 

 ones ; these are brought into position for action by the rising of 

 the cushions upon which they stand. If the ambulacral groove be 

 touched the adambulacral spines come together over it. 



The movements of the tube-feet are co-ordinated by the deep 

 nerve-tracts. At the base of each arm is a centre which normally 

 causes the feet of that arm to point towards its tip, which is the 



