128 AN INTRODUCTION TO THE STUDY OF FOSSILS 



General Survey of Class Anthozoa 



Usually sessile polyps differing from Hydrozoa in the posses- 

 sion of a short oesophagus, differentiated from the digestive 

 cavity, and in the division of the body chamber into radiating 

 compartments by means of the mesenteries. 



Some Anthozoa, as the sea anemone, are without hard parts, 

 but in most forms a skeleton of lime or chitin is secreted by the 

 ectoderm, more rarely by the mesogloea. The simplest form 

 of coral skeleton is that composed of microscopic spicules of 

 lime carbonate of different shapes, developed in great quantities 

 in the mesogloea and remaining detached, as in Alcyonium 

 (" dead men's fingers "). In some forms the spicules are firmly 

 cemented together, thus forming tubes {" organ-pipe coral," — 

 Tubipora) or a branching axis {" precious red coral," — 

 Corallium). In most corals there are no spicules present but 

 a calcareous skeleton is secreted by the outer surface of the 

 ectoderm of the lower part of the polyp. This entire skeleton 

 is called the corallum; when consisting of one individual it is 

 called simple ; when of many, it is termed compound. In the 

 latter case each individual is termed a corallite. In a simple 

 coral, or a corallite, the outer wall {theca) bounds the radiating 

 vertical plates {septa). The depression in the broad end of 

 the coral, called the calyx, was occupied by the base of the polyp 

 and into this it could partly withdraw for protection. In many 

 corals the theca is surrounded by another calcareous layer with 

 ring-like wrinkles, the epitheca; this was secreted by the exter- 

 nal surface of the ectoderm at the base of the polyp. In some 

 forms the partial or complete absence of one of the principal 

 septa caused the formation of a groove or fossida ; it has been 

 suggested that the mesenteries bounding this produced most of 

 the reproductive elements. For the formation of the septa and 

 basal plate see page 126. Projecting upward from the base 

 at the center of the coral is often a little column, the columella; 

 this is secreted by the center of the base of the polyp ; the ecto- 



