FEEDING AND EXCRETION 29I 



little pouches and slung from the aboral wall of the arm. From the 

 pyloric sac a short, conical rectum leads to the anus. It bears 

 interradially two small brown branches, the rectal caeca. The 

 starfish will eat any animal that it can master. Small prey may 

 be taken into the mouth, but usually digestion is performed in 

 a remarkable manner outside the body ; the arms bend round the 

 prey and hold it with their tube-feet, while the stomach is forced 

 out, by contraction of the body-wall compressing the ccelomic 

 fluid, and enwraps the prey. Enzymes are secreted on to the 

 food, and w^hen digestion is complete the stomach is withdrawn 

 by the retractor muscles. In eating bivalves, which are a great 

 part of its food, the starfish brings two arms over one of the valves 

 and three on to the other, and attaches them by the tube-feet ; 

 the valves are then pulled apart, and the stomach is inserted 

 into the shell. The digestive juice is secreted by the cells which 

 line the pyloric caeca. Shells are left behind by the stomach, or 

 rejected through the mouth, very Httle matter being cast out 

 through the anus. The fluid of the water vascular system contains 

 small quantities of ammonia, amino compounds, and urea, 

 and since it is in communication with the surrounding seawater, 

 nitrogen is presumably lost from it. Similar excreta are said 

 also to be got rid of through the walls of the gills, partly in 

 solution, partly as granules carried by amoeboid cells which pass 

 through to the exterior. The rectal csca appear also to excrete 

 waste matter, which passes out by the anus. The starfish is 

 strictly stenohaline (p. 194), being unable to live except in the 

 sea. 



WATER VASCULAR SYSTEM 



The w^orking of the tube-feet is brought about by a peculiar 

 system of tubes, derived during development from the coelom, 

 known as the water vascular system (Fig. 217). This starts at 

 the madreporite as the stone canal, so called because it is strength- 

 ened by calcareous matter. The wall of this canal makes a curious 

 projection into it, Y-shaped in section, with the arms of the Y 

 rolled, so that the surface is greatly increased, and it is ciliated. 

 The lower end of the stone canal joins a ring canal around the 

 mouth, above the peristome, and from this a radial canal runs 

 down each arm, below the ambulacral ossicles. From the radial 

 canals small transverse canals run, one to each tube-foot. The 



