SURVEY OF INVERTEBRATES 



77 



polyp structure exhibited by the Anthozoa, and it is only necessary 

 to relate this to the Corals which are essentially Metridium-like 

 polyps that secrete, under and about themselves, more or less 

 complex skeletons, chiefly of carbonate of lime. As the polyp 

 continues to secrete the coral, it actually pushes itself farther and 

 farther away from the surface to which it became attached. This 

 growth process, together with multiplication by budding, grad- 

 ually builds up considerable masses of coral about larger and larger 

 colonies of polyps — the arrangement of the polyps and the dis- 



mm 



.Tentacle 



Ectoderm 



Encloderm 



Mesentery 



Theca 



Basal plate 



Columella 



Fig. 42. — Coral. A, Skeleton of a young colony of Organ-pipe Coral, 

 Tubipora musica; B, small branch of Red Coral, Corallium riibrum, showing 

 living polyps; C, Sea Pen, Pennatula phosphorea; D, diagrammatic section of 

 a single coral polyp. 



position of the coral being characteristically different in the 

 numerous species of Coral animals. Certain kinds of Corals, acting 

 through long periods of time, are responsible for building not only 

 atolls and islands but also fringing reefs and barrier reefs; the 

 Great Barrier Reef of Australia is over a thousand miles long and 

 fifty broad. (Fig. 42.) 



From the Hydroid polyps to polyps building coral islands, from 

 the tiny medusae of Obelia to the giant Cyanea^ we gain at least a 



