STARFISHES, SEA-URCHINS, ETC. 91 



mouth directly into a large membranous pouch, the cardiac 

 portion of the stomach. By a short constriction the cardiac 

 portion is separated from the part which lies just above, i. e., 

 the pyloric portion of the stomach. From the pyloric portion 

 large, pointed, paired glandular appendages, the pyloric caeca, 

 extend into each ray. Their function is digestive, and some- 

 times they are spoken of as the digestive glands or "livers." 

 The pyloric cceca, as well as the cardiac portion of the stomach, 

 are held in place by paired muscles which extend into each 

 arm. The pyloric portion of the stomach opens above into 

 a short intestine which terminates in the anus. Attached 

 to the intestine is a convoluted many-branched tube, the 

 intestinal c&cum. 



In the angle of each two adjoining rays are paired glandular 

 reproductive organs, which empty by a common duct on the 

 aboral surface. The small bulb-like bladders extending in two 

 double rows on the floor of each ray are the water-sacs, or am- 

 pulla, and each one is connected directly with one of the tube- 

 feet. 



Passing around the alimentary canal near the mouth is a 

 ring-shaped canal from which the radial vessels run out be- 

 neath the floor of each ray and from which a hard tube extends 

 to the madreporic plate. This hard tube is the stone canal, so 

 called because its walls contain a series of calcareous rings, 

 while the circular tube is the ring canal or circum-oral water- 

 ring, from which radiate the radial canals. In some species of 

 starfish there are bladder-like reservoirs, Polian vesicles, which 

 extend interradially from the ring canal. 



The ampullae and tube-feet are all connected with the radial 

 canals. By the contraction of the delicate muscles in the 

 walls of the ampullag the fluid in the cavity is compressed, 

 thereby forcing the tube-feet out. By the contraction of 

 muscles in the tube-feet they are again shortened, while the 

 small disk-like terminal sucker clings to some firm object. 

 In this way the animal pulls itself along by successive "steps." 

 This entire system, called the water-vascular system, is char- 

 acteristic of the branch Echinodermata. In addition to the 

 fluid in the water-vascular system there is yet another body- 



