FRESH-WATER POLYP AND SOME ALLIES 279 



with its crown of tentacles prove the relationship. Besides, 

 examination of the interior reveals the presence of mesen- 

 teries arranged very much as in Metridium. One important 

 structure of Astrangia, however, is never found in the sea 

 anemone, and that is the carbonate of lime "skeleton." 

 This structure at the base of the polyp is more definitely 

 referred to under the name corallite, for it is not a true 



Fig. 147. Colony of coral polyps. (Enlarged) 



skeleton. Several corallites 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. 



Definition of Anthozoa (Gr. anihos, "flower" ; zoa, "ani- 

 mals"). The members of the class Anthozo'a are alike in hav- 

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

 is bilaterally symmetrical, as indicated in the arrange- 

 ment of mesenteries in the gastrovascular cavity. Super- 

 ficially, however, as indicated by the arrangement of the 

 tentacles, there is radial symmetry. Into the single, par- 

 tially divided gastrovascular cavity the mouth opens, and 



