260 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
tube-feet in the ray. This common or radial tube 
itself opens into a circular tube (Fig. 36, e) sur- 
rounding the mouth of the animal (Fig. 36 m). This 
circular tube therefore receives five radial tubes— 
one from each of the five rays—and is likewise in 
communication with a number of membraneous sacs 
(Fig. 36, c, d), resembling in their structure (though 
Fig. 36.—Diagram of ambulacral system of a Star-fish: a, madreporic canal; }, 
inner end; g, outer end of sinus leading to circular neural vessel; #, from which 
radial neural vessels, l, arise; c d, Polian vesicles; f, ampulle; m, oral aperture; 
m. madreporic plate. 
larger in size) those which occur at the base of each 
of the tube-feet. The function both of these sacs 
and of those at the base of each tube-foot is the 
same, namely, that of acting as reservoirs of the 
fluid when this is expelled from the tube-feet. 
