CCELENTERATA I ACTINOZOA. 97 



CHAPTER XIII. 



ACTINOZOA. 



1. GENERAL CHARACTERS OF THE ACTINOZOA. 2. CHARACTERS 

 OF THE ZOANTHARIA. 3. ZOANTHARIA MALACODERMATA. 

 4. ZOANTHARIA SCLEROBASJCA. 5. ZOANTHARIA SCLERO- 



DERMATA. 



CLASS II. ACTINOZOA. The Actinozoa are defined as Coel- 

 enterata with a differentiated digestive sac opening below into 

 the somatic cavity, but separated from it by an intervening ' peri- 

 visceral space,' which is divided into a series of compartments by 

 vertical partitions, or 'mesenteries,' to the faces of which the re- 

 productive organs are attached. 



The Actinozoa (fig. 25), 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 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. 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 

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

 circulation of the digestive fluids contained therein. The sole 

 digestive apparatus in the Actinozoa consists of a tubular 

 stomach- sac, which communicates freely with the outer world 

 by means of the mouth, and opens inferiorly directly into the 

 general body-cavity. In most, the ' perivisceral space ' be- 

 tween the body- walls and the digestive sac is subdivided into 



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