282 GENERAL ZOOLOGY 



Dr. Murray suggested also the probability of atolls being 

 formed without the preliminary stages of fringing and bar- 

 rier reefs. If coral polyps were to attach to a submerged 

 plateau not too deep for them to live on, the small colony 

 would grow upward and extend itself radially, something 

 like the flaring sides of a shallow washbasin. As the mass 

 grows larger the polyps at the center would be killed by the 

 detritus collecting from the broken pieces of coral rock at 

 the wave-beaten margin, and the rock already formed at 

 the center would be worn and scooped out by erosion with 

 coral sand. 



Coral atolls are imperishable bulwarks against the sea, 

 and in some cases have formed the beginning of strips of 

 land sufficiently wide for wild races of men to live upon. 



Definition of Coelenterata (Gr. koilos, ' ' hollow " ; enteron, 

 "intestine ")• Arranged in the order of their increasing de- 

 gree of specialization, the three classes that make up the phy- 

 lum Coelentera'ta are Hydrozoa, Scyphozoa, and Anthozoa. 

 The classes constitute a fairly distinct phylum by including 

 animals with radially symmetrical bodies (at least in ex- 

 ternal appearance) in which there is but one cavity. This 

 cavity, joining the mouth opening, is the digestive cavity, 

 and is not separated from a body cavity, as in higher ani- 

 mals (whence the derivation of the name of the phylum). 

 The body has but two germ layers, — the ectoderm and the 

 endoderm. In this particular it is different from every 

 phylum of animals heretofore described. The characteristic 

 organs of offense and defense are the nettling cells. 



