PHYLUM CCELENTERATA 



137 



b. A Coral Polyp — A strait gi a 



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

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



mm 



m 



Fig. 8*. — Astrangia dance, a cluster of our Northern coral-polyps, resting on 

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



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 



J Fig. 87. — Semi-diagrammatic view 



radial Septa (Fig. 87, //) which of half a simple coral. /, tentacle; 



are built up between the pairs *• m , outh: f 3 ' gu " et; 4 ' mese " ter y: 



r " 5, edge of mesentery ; 6, ectoderm ; 



of mesenteries (4) of the polyp. 7, entoderm; 8, basal plate; q, theca; 



The center of the cup is occu- SvS^ilSiS^JXi 

 pied by a columella (10) formed Bourne.) 



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