96 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



The stomach completes a half-circle of the body cavity in 

 the left-handed direction, as seen from the aboral side, and is 

 then joined by the narrow siphon mentioned above. At this 

 point the intestine begins, and this organ bends sharply towards 

 the mouth in order to pass between the jaws and the posterior 

 lunule. 



In the right half of the body the intestine bends sharply pos- 

 teriorly beside the posterior lunule and runs in a series of broad 

 loops nearly half-way around the body or nearly to the middle 

 line anteriorly. It then bends sharply back on itself, occupying 

 a position between the inner and middle series of supporting 

 plates, and returns to the middle line between the mouth and the 

 posterior lunule. On reaching a position near the middle line, the 

 intestine changes into a narrow tube, the rectum (Plate XXIV, 

 fig. 1, r), which crosses the first portion of the intestine on the 

 aboral side, and then bends sharply to open to the exterior be- 

 tween the anterior border of the posterior lunule and the mouth. 



The siphon referred to above as lying along the inner border 

 of the stomach, is a narrow tube which makes a short passage 

 between the two ends of that organ. By means of this siphon 

 water can be taken in at the mouth and passed back directly to 

 the intestine without interfering with the digestive juices of the 

 stomach. The water thus introduced is said to serve a respira- 

 tory function in furnishing a supply of oxygen to the viscera. 



In both the key-hole urchin and the sand-dollar, the posterior 

 portion of the stomach and the whole of the intestine are com- 

 monly filled with a mass of fine sand and mud, abounding in 

 diatom shells. This is sufficient indication that the food of these 

 animals consists largely of such microscopic particles of organic 

 matter as occur in the mud or adhering to the grains of sand. 

 These particles doubtless include many kinds of living and dead 

 organisms among which diatoms and other minute algae are 

 abundant. 



If a fresh or well preserved sand-dollar be opened by care- 

 fully breaking away the plates of the aboral surface the internal 

 organs are easily studied. The four brownish purple ovaries or 

 spermaries are attached to the aboral plates. After these are 

 removed the alimentary canal lies freely exposed. 



