u8 



MANUAL OF ZOOLOGY 



features of the two sub-classes are, that in the Zoantharia 

 the tentacles and mesenteries are usually very numerous, 

 and are arranged, as a rule, in multiples of five or six, and 





Fig. 56. 



- The common Sea-anemone. 

 (After Emerton.) 



Fig. 57. — Corallium rubrum, portion of 

 colony. Enlarged twice. (After Lacaze- 

 Duthiers.) 



that the tentacles are simple in form ; while in the Alcyo- 

 naria (Fig. 58) the tentacles and mesenteries are always 

 eight in number, and the tentacles are pinnate, i.e., each 

 of them consists of a main stem with two rows of lateral 

 branchlets. 



Only the sea-anemones (with a few exceptions) and a 

 few Madrepore corals remain simple, the rest all giving 

 rise to more or less extensive colonies, of a variety of differ- 

 ent forms, by continuous budding. The structure of the 

 zooids is similar to that of the sea-anemone in all essential 

 respects. In many of the Alcyonaria two forms of zooids 

 are to be distinguished in each colony (dimorphism of the 

 zooids), ordinary zooids, and siphonozooids, which are smaller, 

 and are devoid of tentacles and of gonads. 



