CCELENTERATA : ACTINOZOA. 125 



ing "perivisceral space" which is divided into a series of compart- 

 ments by vertical partitions, or " mesenteries" to the faces of which 

 the reproductive organs are attached. 



The Actinozoa (fig. 40), therefore, differ fundamentally from 

 the Hydrozoa in this, that whereas in the latter the digestive 

 cavity is identical with the somatic cavity, in the former there 

 is a distinct digestive sac, which opens, indeed, into the 

 somatic cavity, but is, nevertheless, separated from it by an 

 intervening perivisceral space. As a result of this, the body of 

 a typical Actinozoon (fig. 39), exhibits on transverse section two 

 concentric tubes, one formed by the digestive sac, the other by 

 the parietes of the body whereas the transverse section of a 

 Hydrozoon exhibits but a single tube, formed by the walls of 

 the combined digestive and somatic cavity. 



Histologically, the tissues of the Actinozoa are essentially the 

 same as those of the Hydrozoa, consisting of the two funda- 

 mental layers, the " ectoderm " and the " endoderm." In the 

 Actinozoa, however, there is a much greater tendency to a 

 differentiation of these into specialised structures, and in some 

 members of the class muscular fibres are well developed. 

 Thus, the Sea-anemones have a well-developed series of 

 longitudinal and circular muscular fibres, of which the former 

 become radial in the disc and base. The ectoderm, 

 especially, shows a tendency to break up into two layers, 

 which are differentiated in opposite directions from an 

 intermediate zone, and are termed by Huxley the " ecderon " 

 and "enderon," corresponding respectively to the epidermis 

 and derma of man. Cilia are often present, especially in the 



Fig. 39. A, Transverse section of an Actinozoon. a. Digestive sac ; b Wall of the 

 body ; m Mesenteries connecting the stomach with the body-walls, and dividing the 

 space between them into a number of vertical compartments. B, Transverse section 

 of a. Hydrozoon, showing the single tube formed by the walls of the body. 



interior of the somatic cavity, where they serve to promote a 

 circulation of the digestive fluids contained therein. The sole 



