PHYLUM CCELENTERATA 



137 



b. A Coral Polyp Astrangia 



Astrangia dance (Fig. 86) is a coral polyp inhabiting the waters 

 of our North Atlantic coast. A number of individuals live to- 



FIG. 86. Astrangia dance, a cluster of our Northern coral-polyps, resting on 

 limy bases of their own secretion. (From Davenport, after Sourel.) 



gether in colonies attached to rocks near the shore. Each polyp 



looks like a small sea-anemone, being cylindrical in shape and 



possessing a crown of tentacles. 



The most noticeable difference 



is the presence of a basal cup 



of calcium carbonate termed 



the theca (Fig. 87 p). This 



structure of calcium carbonate 



is what we commonly call 



coral. It is produced by the 



ectoderm of the . coral polyp 



and increases gradually during 



the life of the animal. 



The calcareous cup is divided 

 into, chambers by a number of Fm g? _ Semi . diagrammatic view 



radial septa (Fig. 87, II) which of half a simple coral, i, tentacle; 



are built up between the pairs J ^ ^^ . * ; 



of mesenteries (4) of the polyp. 7, entoderm; 8, basal plate; Q, the.ca; 



The renter of the rim is orru I0 ' columella ' JI > septum. (From 



3CC ~ U ~ Shipley and MacBride, partly after 



pied by a c olumella (10) formed Bourne.) 



