86 



CCELENTERATA. 



or "corallum." All these groups, therefore, are known as 

 fossils ; but they are of very unequal importance. The Zoan- 

 tharia sclerobasica and Alcyonaria are known by very few fossil 

 representatives, and require to be little more than mentioned. 

 The Zoantharia sclerodcrmata and Rugosa, on the other hand, 

 have left very numerous and interesting traces of their former 

 existence the latter being almost altogether extinct, and 

 both will require to be noticed at some length. Regarded 

 as a whole, the class of the Actinozoa appears, so far as we yet 

 know, to have commenced its existence in the Upper Cam- 

 brian period, and to have attained its maximum 6f develop- 

 ment at the present day. 



ORDER I. ZOANTHARIA. 



Tentacles simple, rounded ; soft parts in multiples of five or six. 



Sub-order I. Zoantharia malacodermata. Ex. Sea-anemone. 

 ,, 2. Zoantharia sclerobasica. Ex. Antipathes. 

 ,, 3. Zoantharia sclerodermata. Ex. Reef-building Corals. 



A. ZOANTHARIA MALACODERMATA. Though, from their soft 

 nature, unknown in a fossil condition, the Sea-anemones merit 

 a brief description here, as they may be taken as the types of 

 the order, and as the somewhat complicated structure of the 

 sclerodermic coral will thereby be rendered much more intel- 

 ligible. 



Fig. 36. A, Actinia mesentbryantliemutti, 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 body of a Sea-anemone (fig. 36) is a truncated cone, or 

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



