STAR-FISH AND SEA-URCHINS. 261 
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 walls of the tube-feet; that is to 
say, when the muscles of the reservoirs contract 
(Fig. 36, ¢,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. 32, m, and Fig. 36, 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, with 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 which 
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 madreporic tu- 
bercle, on arriving at which it slowly oozed through 
the porous substance of which that tubercle consists. 
