COEAL POLYPS. — STONY COEALS. 173 



Through this common base the cavities of the polyps 

 communicate, more or less directly, so that food obtained 

 by one may nourish the whole colony. The coral polyps 

 also differ from the sea-anemone in forming a deposit of 

 hard matter. Representatives of the two kinds of coral 

 should now be examined. 



STONY CORALS. 



{Corals Proper.^) 



In a piece of stony coral, or compound skeleton of a 

 colony of coral polyps (Gralazea is a good form to study), 

 make out the following points : — 



1. The nature of the material itself; test by putting a 

 very small piece into weak acid, or by touching the 

 specimen with a drop of acid. 



2. The cup, or theca, formed by an individual polyp, 

 often traceable as a long tube. Observe, — 



a. The outer wall of the cup. 



b. The partitions, or septa, extending inward from 

 the wall of the cup. 



3. Between the cups, the porous limy secretion, which 

 was secreted by the common body substance, or cce- 

 nosarc, connecting the individual polyps. 



Imagine the sea-anemone depositing limy matter in 

 the base of its body wall, forming a cup; fleshy radial 

 ridges rising from the floor and wall of the cup be- 

 tween the mesenteries, and a similar deposit in these 

 ridges; thus it will be seen how the cup is formed by 



