ACTINOZOA. 



241 



ism consisting of a single " polype " ; or they may be composite, the 

 organism being composed of more or less numerous polypes con- 

 nected with one another directly, or united by a common " cceno- 

 sarc." 



Taking a simple Actinozoon, such as a Sea-anemone (fig. 121), 

 as the type of the class, the body is seen to form a cylindrical tube, 

 the walls of which are 

 formed by the three layers 

 above spoken of, enclos- 

 ing an internal cavity 

 (the "visceral chamber"). 

 The base of the cylinder 

 is usually completely 

 closed, and often forms 

 a muscular disc of attach- 

 ment (" pedal disc "). 

 The distal end of the 

 cylindrical body, on the 

 other hand, is perforated 

 centrally by the oval fis- 

 sure of the mouth, and 

 is furnished round its 

 margin with a series of 

 hollow, muscular and tac- 

 tile "tentacles." The 

 mouth opens into a mem- 

 branous oesophageal tube, 

 formed by an infolding 

 of the ectoderm and 

 endoderm, which hangs 



Fig. 120. — A polype of Astroides calyctilaris, a recent 

 Zoantharian, vertically divided. /, Tentacles ; g, CEso- 

 phagus ; iv, Body-wall ; me, Flat face of a mesentery; /, 

 Edge of a mesentery; s/>, One of the calcareous "septa" 

 of the corallum, intervening between the mesenteries ; r, 

 Reproductive organ ; ca, Corallum; co, Columella; cot, 

 Ccenosarc. (After Lacaze-Duthiers.) 



down into the body- 

 cavity, and terminates at 

 some distance above the 

 proximal extremity of the 

 animal in a wide aperture by which it communicates freely with 

 the general cavity of the "visceral chamber." 



The general space ("visceral cavity") included within the body- 

 walls is subdivided into radiating compartments by a series of ver- 

 tical membranous partitions, which spring from the body -wall 

 internally and are directed inwards, constituting what are known 

 as the " mesenteries." Some of these partitions — known as the 

 "principal" or "primary" mesenteries — extend all the way from 

 the body-wall to the side of the oesophagus ; and in the Akyonaria 

 all the mesenteries do so. In most cases, however, there are shorter 

 mesenteries, which fall short of the gullet, and which are known as 



VOL. I. Q 



