CCELENTERATA I ACTINOZOA. I 2 7 



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

 cesses as in the Hydrozoa. Sexual reproduction occurs in all 

 the members of the class, but in many forms gemination 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 "aetinosoma," or entire bodyofany^<r//;/0m>', 

 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 organism. 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." 



The Actinozoa are divided into four orders viz., the Zo- 

 antharia, the Alcyonaria, the Rugosa, and the Ctenophora ; 

 but the last is sometimes placed amongst the Hydrozoa, and 

 it has been recently proposed to remove the Rugosa also to 

 the same class. 



ORDER I. ZOANTHARIA. The Zoantharia or " Helianthoid 

 Polypes" are defined by the disposition of their soft parts in 

 multiples of five or six, and by the possession of simple, usually 

 numerous, tentacles. There may be no corallum, or rarely a 

 " sclerobasic" one. Usually there is a " sclerodermic " corallum, 

 in which the septa in each corallite, like the mesenteries, are 

 arranged in imdtiphs of five or six. 



The Zoanmaria are divided into three sub-orders the Zoan- 

 tharia malacodermata, the Z. sclerobasica, and the Z. scleroder- 

 mata; according as the corallum is entirely absent or very 

 rudimentary, is " sclerobasic," or is " sclerodermic." 



SUB-ORDER I. ZOANTHARIA MALACODERMATA. In this sec- 

 tion of the Zoantharia there is either no corallum or 3, Very 

 rudimentary one, in the form of a few scattered spicules. The 

 "actinosoma" is usually composed of but a single polype. 



