150 PROCEEDINGS OF THE AMERICAN ACADEMY. 



is here an unimportant one, as the feeding reactions of the various 

 members of the genus are doubtless essentially the same. 



The rose corals at Bermuda form small convex colonies, seldom 

 measuring more than five or six inches in diameter. They occur abun- 

 dantly in shallow water near shore, often in quiet bays partially shut 

 oif from the open sea, where the plankton, which apparently furnishes 

 them with food, is considerably diminished in quantity. Their suc- 



T 5i> 



Figure 1. Diagrammatic vertical section through an expanded polyp. 



C, coelenteron or gastrocoelomic ca\aty ; EZ, edge zone ; M, mesentery; 

 MF, mesenterial filament ; OD, oral disk ; Sp, sphincter muscle ; Si, stomo- 

 daeum ; T, tentacle. 



cessful adaptation to these apparently unfavorable situations, in which 

 most corals do not thrive, suggests a very efficient mechanism for 

 availing themselves of such food as may be present. 



The individual polyps of the rose-coral colony are comparatively 

 large, the oral surface often measuring an inch in diameter. The 

 structural features of this surface are shown in the Plate, which is from 

 a photograph of a small portion of a partially expanded colony. In 

 the center of the polyp is the mouth (M), a small oval or circular aper- 

 ture, which opens into the stomodaeum. Bordering the mouth is the 

 flat ciliated oral disk (OJ)). This is bounded by a circle of short 

 tentacles (7^, external to which is a thickened ciliated edge zone (J'-^Z), 

 raised somewhat above the level of the oral disk, and forming the wide 

 rim which completes the oral surface peripherally. A diagrammatic 

 vertical section through an expanded polyp (Figure 1) reveals its cup- 



