STAR-FISH AND SEA-URCHINS. 2G1 



Moreover, all these membraneous sacs are provided 

 with ring-shaped muscular fibres in their mem- 

 braneous walls, which therefore serve as antagonists 

 to the ring-shaped muscles which occur in the 

 membraneous Avails of the tube-feet ; that is to 

 say, when the muscles of the reservoirs contract 

 (Fig. 3G, c, d,f\ ), the pressure in the tube-feet is in- 

 creased, and when these muscles relax, that pressure 

 is diminished. The animal is thus furnished with 

 the means of varying the head of pressure in its 

 tube-feet, either locally or universally. 



The circular tube surrounding the mouth com- 

 municates at one point with a calcareous tube 

 (Fig. 36, a), which runs straight to the dorsal surface 

 of the animal, and there terminates in the madre- 

 poric tubercle, to which I have already directed 

 attention (Fig. 82, m, and Fig. r3G, m). Thus it will 

 be seen that all the pedicels of all the rays are in 

 communication, by means of a closed system of 

 tubes, w^ith this madreporic tubercle. It has there- 

 fore been surmised that the function of this tubercle 

 is that of acting as a filter to the sea-water wdiich 

 in large part constitutes the fluid that fills the 

 ambulacral system. We have been able to prove 

 that this surmise is correct ; for we found that if 

 we injected any part of the ambulacral system with 

 coloured fluid — maintaining the injection for several 

 hours at as great a pressure as the tubes would 

 stand without rupturing — the coloured fluid found 

 its way up the calcareous tube to the ma Ireporic tu- 

 bercle, on arriving at which it slowly oozed through 

 the porous substance of whicli that tubercle consists. 



18 



