VIII 



ECHINODERMATA 



287 



object. The longitudinal fibres in the wall of the tube-foot then contract 

 so that it shortens and pulls the body of the starfish towards the point 

 of attachment. 



For the efficient working of the tube-feet it is clearly necessary that 

 the hydrocoele should be distended with fluid : but it is also clear that, 

 the walls of the tube-feet and ampulla being comparatively thin, a 

 certain amount of loss of fluid must take place by a process of filtration 

 through them. A special mechanism is therefore necessary to compensate 

 for this loss of fluid. On the aboral surface of the starfish and situated 



a,.n. 



Transverse section through arm of a starfish. (Simplified from Lang.) a, Ampulla ; a.n, aboral 

 (apical) nerve ; a.o, ambulacral ossicle ; p, papula (gill) ; p.c, pyloric caecum ; ph, perihaemal space ; 

 r.c, radial canal of hydrocoele ; r.n, radial nerve ; s, spine ; t.j, tube-foot. 



interradially i.e. in the angle between the axes of two adjacent arms 

 is a conspicuous ossicle (Figs. 116, A ; and 117, m), its surface indented 

 by meandering grooves dotted along the bottom of which are minute 

 pores. This ossicle is the madreporite. From its under side there passes 

 downwards to the circum-oral ring of the hydrocoele a tube (Fig. 117, s.c) 

 known as the stone-canal from the fact that its walls are so infiltrated 

 with calcium carbonate as to have a hard stony consistency. This 

 stone-canal is lined with epithelium carrying powerful cilia or flagella, 

 the beating of which causes a slow downward current water being 

 drawn in by the pores of the madreporite and propelled onwards to the 

 ring canal and thence to the rest of the hydrocoele to make up for the 

 loss of fluid. 



