90 



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



After making a nearly complete circle, the intestine passes grad- 

 ually into the rectum, which bends obliquely upward and opens 

 to the exterior in the center of the aboral surface. 



The siphon makes a short cut between the esophagus and the 

 posterior end of the stomach. It consists of a narrow tube im- 

 bedded in the inner wall of the stomach, and is lined throughout 

 with a strongly ciliated epithelium, which presumably draws a 

 stream of water from the esophagus to the intestine. This water 

 doubtless aids in respiration, and may also serve to wash indi- 

 gestible food materials out of the alimentary canal. 



The food of the sea-urchin consists of seaweeds and various 

 small animals. Some species devour such dead animal matter as 

 can be obtained, individuals sometimes collecting in numbers on 

 the body of a dead fish. The organic matter on the surface of 

 the sea bottom is also used for food, while certain species are 

 said to capture and devour large crustaceans. The sharp chisel- 

 like teeth would seem to fit the creature for devouring almost any 

 kind of plant or animal matter which by its slow method of loco- 

 motion it is able to secure. 



Water-vascular System. — From the madreporic plate, the 

 madreporic canal leads directly down to the circular vessel, which 

 is situated immediately above the jaws and surrounds the base 

 of the esophagus. Connected with the circular vessel are five 

 interradial pouches, or Polian vesicles (Plate XXII). 



Five radial vessels, similar to those of the starfish, leave the 

 circular vessel, pass to the borders of the peristome and thence 

 along the inside of the test directly beneath the ambulacral areas, 

 and end in the terminal tentacle of the ocular plate. The radial 

 vessels supply the tube-feet as in the starfish, each tube-foot con- 

 necting with an ampulla inside the wall of the test (Plate XXII). 

 Two perforations of the plates of the test are provided for each 

 tube-foot, so that the contained fluid may enter the foot by one 

 canal and return to the ampulla by the other. Internal cilia keep 

 up this circulation of fluid, and by this means the fluid in the 

 tube-feet is continually changing. An exchange of gases can 

 take place between the fluid contained in the tube-feet and the 

 sea water, and a supplementary respiration is accomplished 

 thereby. In Arbacia only the tube-feet of the lower part of the 

 body are provided with suckers and used for locomotion, those 



