

iv PHYLUM CCELENTERATA 115 



3. THE ACTINOZOA 



The simplest and most familiar of the Actinozoa are the 

 Sea-anemones, which are to be found attached to rocks, 

 sea-weeds, shells, etc., on the sea-shore. When expanded 

 a Sea-anemone has the form of a cylindrical column at- 

 tached to a rock or other support by a broad base. The 

 distal or free surface of the column, termed the disc or 

 peristome, bears in the middle an elongated slit-like aper- 

 ture, the month. Springing from the disc and encircling 

 the mouth are numerous cylindrical tentacles, disposed in 

 circlets, their total number being some multiple of five. 



Obviously the Sea-anemone is a polype, formed on the 

 same general lines as a polype of the Hydrozoa. But cer- 

 tain important differences from the Hydrozoan polype 

 become manifest when we examine the internal structure 

 (Fig. 52). The mouth does not lead at once into a spacious 

 undivided enteric cavity, but into a short tube (*/.), having 

 the form of a flattened cylinder, which hangs downwards 

 into the interior of the body, and terminates in a free edge. 

 This tube is called the gullet or stomodaeum. Its inner 

 surface is marked with two longitudinal grooves (sgph.\ 

 known as the gullet-grooves or siphonoglyphes. The gullet 

 does not simply hang freely in the interior cavity, but is 

 connected with the body-wall by a number of radiating 

 partitions, the complete or primary mesenteries (mes. i) : 

 between these are incomplete secondary mesenteries (mes. 2), 

 which extend only part of the way from the body-wall to 

 the gullet, and tertiary mesenteries (mes. 3), which are 

 hardly more than ridges on the inner surface of the body- 

 wall. Thus the entire enteric cavity of a sea-anemone is 

 divisible into three regions : (i) the gullet or stomodaeum, 



I 2 



