260 GENERAL ZOOLOGY 



at the base of the polyp is more definitely referred to under 

 the name corallite, for it is not a true skeleton. Several coral- 

 lites are shown in the lower portion of the figure. The hard, 

 radial plates repeat the number of pairs of mesenteries of the 

 polyp, since each plate is formed between the members of 

 pairs of mesenteries. 



The aboral surface of the polyp is molded over the corallite 

 so that the soft tissue of the animal extends down between 

 the radial plates, and also over the circumference. Although 

 the polyp may be said to fit into the corallite, it is the real 

 cause of the existence of the corallite. 



The substance of the corallite is formed by the crystalli- 

 zation of carbonate of lime in the epidermal cells of the polyp. 

 Crystallization takes place in the outermost cells, and as the 

 substance in them hardens newer cells are formed just within, 

 and in these the process of crystallization continues. Thus it 

 is that as the coral polyp grows, it actually pushes itself far- 

 ther and farther away from the surface to which it became 

 attached at the beginning of its fixed life. Astrangia has a 

 characteristic method of forming a colony. The height of a 

 corallite does not exceed a half-inch. The colony increases 

 by budding from the older polyps at their bases. The result 

 is a low mass of corallites that seldom exceeds an area of four 

 inches square. 



Definition of Actinozoa (Gr. aktis (aktiri), ray ; zoon, ani- 

 mal). The members of the class Actinozo'a are alike in having 

 a soft, generally cylindrical body. The plan of structure is 

 bilaterally symmetrical, as indicated in the arrangement of 

 mesenteries in the gastrovascular cavity, it being possible to 

 pass a line lengthwise through the mouth to opposite siphono- 

 glyphes, and separate one half the mesenteries from the other 

 half. Superficially, however, as indicated by the arrangement 

 of the tentacles, there is radial symmetry. Into the single, 

 partially divided gastrovascular cavity the mouth opens, and 



