334 ECHINODERMATA [CH. 



From the ring canal also in one interradius, where the tenth 

 Tiedemann's body if it existed would be found, a tube is given off 

 which leads to the upper surface of the disc, where it opens by a 

 sieve-like plate, pierced by numerous minute pores, called the 

 madreporite (Figs. 151 and 152). This vertical tube receives the 

 awkward name of the stone-canal because its walls are stiffened 

 by calcareous deposit ; its cavity is reduced to a mere slit by the 

 projection into it of an outgrowth of its wall shaped in section 

 like a Y with coiled ends, which is also strengthened by lime. 

 Although, as we have said, the cavity of the stone-canal is a mere 

 slit, yet it is lined by long narrow cells carrying most powerful 

 cilia. In many species of star-fish, although not in Aster ias, stalked 

 sacs resembling greatly enlarged ampullae are attached to the water- 

 vascular ring. These appear to act as reservoirs of fluid for it: 

 they are known as Polian vesicles after Poli, the naturalist who 

 first described them. 



Now since all the movements of a tube-foot can be accounted for 

 by the action of the longitudinal muscles of its lower part and the 

 circular muscles of the ampulla, the question arises as to what is 

 the purpose of this apparatus of radial and circular tubes, stone- 

 canal and madreporite. There is one interesting little mechanism 

 which supplies a valuable clue to the answer to this question. This 

 is a pair of valves placed in the tube-foot at the entrance of the 

 transverse canal, which unites it with the radial tube. These valves 

 swing open into the tube-foot when the pressure in the radial tube 

 is greater than the pressure in the tube-foot, but when the pressure 

 in the latter is higher they close, so that under no circumstances 

 can water escape from the tube-foot into the radial canal. So it 

 appears that there is an arrangement which allows fluid to pass into 

 the tube-foot but which prevents its return, and this implies that 

 under ordinary circumstances there must be a loss of fluid from the 

 tube-foot. We must in fact suppose that when the tube-foot is 

 driven out by the contraction of the ampulla, the contained fluid 

 slowly transudes through its thin walls and the loss is supplied from 

 the radial canal. The pressure in the radial and circular canals is 

 kept up by the action of the cilia in the stone-canal, by means of 

 which a slow but steady current is produced, setting in from the 

 outside through the madreporite. 



The function of the whole system of tubes therefore is to keep 

 the tube-feet full of fluid and thus tense and rigid, so that they can 

 perform their functions properly. 



