I IS 



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. 



