CCELENTERATA : ACTINOZOA. I^I 



acterislic of the Lower Silurian period. In Didymograptus 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." 



The Hydrocorallina. with the exception of some Cretaceous forms allied 

 to Millepora, are not known to be represented in deposits older than the 

 Tertiary. 



CHAPTER XII. 



ACTINOZOA. 



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

 ACTERS OF THE ZOANTHARIA. 3. ZOANTHARIA MALA- 

 CODERMATA. 4. ZOANTHARIA SCLEROBASICA. 5. ZOAN- 

 THARIA SCLERODERMATA. 



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

 terata 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 of which 

 the reproductive organs are attached. 



The Actinozoa (fig. 69), 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 Actinozob'n (fig. 68) 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. Cilia are often present, 



