CCELENTERATA. ACTINOZOA. 51 



at its lower end into the body-cavity (d). The latter is 

 divided into chambers by folds of the endoderm, the mesen- 

 teries (e) ; in the upper part of the polyp, their inner 

 edges join the oesophageal tube, in the lower part they 

 remain free, so that a section in the former region (fig. 

 10 B} will show the body wall and also the oesophageal 

 wall, but in the latter (G) the body wall only. The tenta- 

 cles are placed immediately above the intermesenteric 

 chambers, and the space in each tentacle is continuous 

 with that of the chamber below. In Actinaria as in the 

 other sea-anemones there is no skeleton, but the majority 

 of actinozoans possess one, which in many cases is external 

 to the body, being formed of carbonate of lime, secreted 

 by the ectoderm ; this is said to be sclerodermic : in others 

 it is formed internally and consists of calcareous spicules, 

 or there is a coenosarcal axial rod of horny or calcareous 

 material, this last being a sclerobasic skeleton. The Acti- 

 nozoa can be divided into three orders, (1) Zoantharia, 

 (2) Alcyonaria, (3) Ctenophora. The Monticuliporoidea 

 may also perhaps belong to this class, but they are con- 

 sidered by several authors to be Polyzoa. 



OBDEK. ZOANTHAKIA. 



In the Zoantharia, the tentacles and mesenteries are 

 generally numerous and are never eight in number as is 

 the case in the Alcyonaria, but frequently a multiple of 

 six or sometimes four. The tentacles are always simple. 

 A skeleton is usually present and it may be calcareous 

 or horny. 



There are three sub-orders (1) the Actinaria, including 

 the sea-anemones, which possess no skeleton and are 

 consequently unknown in the fossil state ; (2) the Anti- 



42 



