PHYLUM CCELENTERATA 139 



An atoll (Fig. 88) is a more or less circular reef enclosing a 

 lagoon. Several theories have been advanced to account for the 

 production of atolls. Charles Darwin, who made extensive 

 studies of coral reefs and islands, is responsible for the subsidence 

 theory. According to Darwin, the reef was originally built up 

 around an oceanic island which slowly sank beneath the ocean, 

 leaving the coral reef enclosing a lagoon. John Murray be- 

 lieves that the sland enclosed by the reef does not necessarily 

 sink, but may be worn down by erosion. 



Besides producing islands and reefs, corals play an important 

 role in protecting the shore from being worn down by the waves. 

 They have also built up thick strata of the earth's crust. 



d. Classification of the Anthozoa 



The Anthozoa may be divided into two subclasses and ten 

 orders. 



Subclass I. Alcyonaria. — Anthozoa with eight hollow, 

 pinnate tentacles, and eight complete mesenteries; with one 

 siphonoglyphe, ventral in position ; and with the retractor muscles 

 of the mesenteries all on the side toward the siphonoglyphe. 



Order 1. Stolonifera. — Alcyonaria colonial in habit; with 

 stolon attached to a stone or other foreign object; polyps free 

 except at base or joined together by horizontal bars; skeleton 

 either horny or of calcareous spicules. Example: Tubipora 

 (Fig. 89, A). 



The organ-pipe coral, Tubipora (Fig. 89, A), is common on 

 coral reefs. It has bright green tentacles and a skeleton of a dull 

 red color, and adds considerably to the beauty of the coral reef. 



Order 2.. Alcyonacea. — Colonial Alcyonaria; zooids united 

 into a compact mass by fusion of body-walls ; skeleton of 

 calcareous spicules which do not form a solid axial support. 

 Example: Alcyonium (Fig. 89, B). 



Order 3. Gorgonacea. — Colonial Alcyonaria; skeletal 

 axis branched and not perforated by gastrovascular cavities of 

 the zooids. Example: Cor allium (Fig. 89, C). 



