Il6 MANUAL OF ZOOLOGY. 



A nervous system has not yet been proved to exist in any of 

 the Actinozoa, except in the Ctenophora, and in none are there 

 any traces of a vascular system. 



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 jffydrozoa. Sexual reproduction occurs in all 

 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 Hy- 

 drozoa. 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 J and one order, the Ctenophora, is composed of highly 

 active, free-swimniing organisms. Some of the Actinozoa are 

 unprovided 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 par- 

 tially 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 ar- 

 ranged in multiples of five or six. 



The Zoantharia 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 veiy 

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



