NO. 19.] ECHINODERMS OF CONNECTICUT. 89 



pillars, the auricles, to which are attached the powerful muscles 

 for protruding and retracting the jaws. In Strongylocentrotus 

 the pillars of each pair are connected by a bridge. 



Internal Anatomy. — If the test is cut through at the equa- 

 torial region, and the two portions laid side by side without 

 injuring the alimentary canal, the internal organs of the sea- 

 urchin are easily studied. The more prominent organs to be 

 seen in such a dissection are shown on Plate XXII. 



Digestive System. — In the sea-urchin, the mouth, situated 

 in the center of the oral surface, is provided with five sharp cal- 

 careous teeth which meet closely in the axis of the body and 

 close the mouth completely. These teeth are set in jaws which 

 are moved by a complicated set of muscles attached to a cal- 

 careous frame-work ; this, with the attached teeth, is often called 

 the lantern. When the jaws are separated the mouth presents a 

 wide opening leading into the esophagus. The latter is short, 

 and at its end divides into two branches, of which one is coiled 

 irregularly for some distance and opens into the wide, saccular 

 stomach, while the other forms the siphon (Plate XXII). 



The stomach is broad and flat; its walls are irregularly con- 

 stricted to form numerous sac-like pouches which greatly increase 

 the surface area. It makes a nearly complete circle at the equator 

 of the body and then bends sharply upward and passes into the 

 narrower intestine. The intestine is also flat, and at its beginning 

 bends directly backwards and encircles the body. Its course lies 

 parallel with the stomach but it leads in the opposite direction. 



Explanation of Plate XXII. Anatomy of Sea-urchin. Arbacia 

 punctulata, showing the arrangement of the internal organs. The test 

 has been cut horizontally and the two parts laid side by side without 

 cutting the alimentary canal; the upper circle representing the aboral 

 portion of the test and the lower circle the oral part. At the center of 

 the lower section the mouth opens into the esophagus O) from which 

 the spihon (siph) and stomach (st) branch out. The intestine (int) in 

 the upper section lies parallel with the stomach in its natural position, 

 but its course is in the opposite direction. The rectum (r) opens through 

 the anus at the center of the upper section. The five sexual glands ( s. g) 

 open by narrow ducts on the aboral surface near the anus. The five teeth 

 (/) lie at the five angles of the mouth. The ring canal (r. c) with the 

 five interradial pouches (i p) are shown. Beside the esophagus lies the 

 delicate madreporic canal with its dorsal ampulla (s. c) although the con- 

 nection of the latter with the aboral surface has necessarily been torn 

 away. The rows of ampullae {amp) and tube-feet (t. f) connected with 

 the radial canals are shown in both sections. 



