114 MANUAL OK ZOOLOGY. 



grapsus 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. 



3lass II. Actinozoa. — The Actinozoa are defined as Cxl- 

 tmterata with a differentiated digestive sac opening below into the 

 somatic cavity, but separated from the body-walls by an interven- 

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

 ments by vertical partitions, or " mesenteries," to the faces ofwMch 

 the reproductive organs are attached. 



The Actinozoa (fig. 30), 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. 29), 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 epidennis 

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

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



