CCELENTERATA : ACTINOZOA. 



153 



Distinct reproductive organs occur in all the Actinozoa, but 

 these are internal, and are never in the form of external pro- 

 cesses as in the Hydrozoa. Sexual reproduction occurs in all 



E'ig. 69. A, ^Actinia mesembryanthemum, one of the Sea-anemones (After Johnston); 

 B, section of the same, showing the mouth (a), the stomach (b), and the body- 

 cavity (c). 



the members of the class, but in many forms gemmation or 

 fission constitutes an equally common mode of increase. 

 Some Actinozoa, therefore, such as the common Sea-anemones, 

 are simple organisms ; whilst others, such as the reef-building 

 corals, are composite, the act of gemmation or fission giving 

 rise to colonies composed of numerous zooids united by a 

 ccenosarc. In these cases the separate zooids are termed 

 " polypes," the term " polypite " being restricted to the 

 Hydrozoa. In the simple Actinozoa, however, the term 

 "polype "is employed to designate the entire organism. In 

 other words, the "actinosoma," or entire body of any Actinozoon, 

 may be composed of a single " polype," or of several such, 

 produced by a process of continuous gemmation or fission, and 

 united by a common connecting structure, or ccenosarc. 



Most of the Actinozoa are permanently fixed ; some, like the 

 Sea-anemones, possess a small amount of locomotive power ; 

 and one order, the Ctenophora, is composed of highly active, 

 free - swimming organisms. Some of the Actinozoa are un- 

 provided with any hard structure or support, as in the Sea- 

 anemones and in all the Ctenophora; but a large number 

 secrete a calcareous or horny, or partially calcareous and 

 partially horny, framework or skeleton, which is termed the 

 " coral," or " corallum." 



