CCELENTERATA : ACTINOZOA. Ill 



is employed to designate the entire organism. In other words, 

 the " actinosoma," or entire body of any Actinozob'n, maybe 

 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 coenosarc. 



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 

 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 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 multiples of five or six. 



The Zoantharia are divided into three sub-orders, the Zoan- 

 tharia Malacodermata, the Z. sclerobasica, and the Z. sckroder- 

 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 a very ru- 

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

 " actinosoma " i-s usually composed of but a single polype. 

 (The term " actinosoma " is a very convenient one to express 

 in the Actinozoa what "hydrosoma" expresses in the Hydrozoa^ 

 namely, the entire organism, whether simple or compound.) 



There are rftree families in this section, of which the Acti- 

 nida will require a somewhat detailed examination, since they 

 may be taken as typical of the entire class of the Actinozoa. 



FAMILY I. ACTINID^E. The members of this family are 

 commonly known as sea-anemones, and are distinguished by 

 having no evident corallum, by being rarely compound, and 

 by having the power of locomotion. 



The body of a sea-anemone (fig. 29, a) is a truncated cone, 

 or a short cylinder, termed the "column," and is of a soft, 



