CCELENTERATA : ACTINOZOA. IOQ 



period. In Didymograpsus the polypary consists of two 

 lateral symmetrical branches, with cellules on one side only, 

 springing from a central point or base, which is usually marked 

 by a little spine or " radicle." 



CHAPTER XIII. 



ACTINOZOA. 



i. GENERAL CHARACTERS OF THE ACTINOZOA. 2. CHAR- 

 ACTERS OF THE ZOANTHARIA. 3. ZOANTHARIA MALACO- 

 DERMATA. 4. ZOANTHARIA SCLEROBASICA. 5. ZOAN- 

 THARIA SCLERODERMATA. 



CLASS II. ACTINOZOA. The Actinozoa are denned as Ccel- 

 enterata with a differentiated digestive sac opening below into 

 the somatic cavity, btit separated from the body-walls by an inter- 

 vening "perivisceral space" which is divided into a series of com- 

 partments by vertical partitions, or " mesenteries" to the faces of 

 which the reproductive organs are attached. 



The Actinozoa (fig. 28), 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 con- 

 centric 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 



