172 GENERAL ZOOLOGY 



one of the centers are removed the whole umbrella keeps 

 on beating as before ; if all the centers are removed, however, 

 there is no more pulsation. The medusa therefore has a 

 very wide margin of safety in case of injury. 



CLASS 3. ANTHOZOA 



The Anthozoa have no medusa stage, there is, in fact, no 

 indication of metagenesis. The polyps produce eggs and 

 spermatozoa which form zygotes, and these grow into 

 polyps like their parents (Fig. 71, D). Anthozoan polyps 

 are more complicated than those in any other group. 

 There is a gullet leading from the mouth into the digestive 

 cavity, which is divided up into small compartments by 

 thin partitions. 



The anemones are solitary polyps which form no skeleton 

 whereas corals are colonial anthozoans. Corals live in 

 large colonies, often showing great polymorphism, and 

 secrete a skeleton of lime or other substances. Many 

 islands and atolls have been formed entirely from the skele- 

 tons of corals during periods when the bottom of the ocean 

 was slowly sinking. 



GENERAL REMARKS ON OELENTERATES 



When considered in relation to the Protozoa and Porifera, 

 the phylum Ccelenterata is remarkable for several reasons. 

 The following points should be particularly kept in mind: 

 (1) though many Ccelenterata are very complicated in struc- 

 ture, none is much more than a modified gastrula; (2) 

 though tissues are rather poorly developed in some, others 

 have well-defined supporting, nervous and muscular tissues; 

 (3) organs are present; (4) a definite cavity, the enteron, is 

 used for digestion and it presents the simplest condition to 

 be found in any of the Enteroccela (Fig. 18, page 24), having 

 but one opening; (5) colony formation and polymorphism 

 are common ; (6) metagenesis, or alternation of a sexual and 



