DIVISION OR TYPE VI. : COZLENTERATA. 



Eadially symmetrical, four or six rayed animals possessed of a single 

 internal cavity, which represents the body cavity, alimentary canal, and 

 blood-vascular system of higher animals. 



Before considering the separate divisions of the Ccelenterata, we shall 

 examine, by way of "an introduction to the structure of this wide- 

 branching division of the animal kingdom, a species easily understood 

 and accessible, viz. : 



The Fresh-Water Polyp (Hydra). 



A. Occurrence and Mode of Capture. 



If from a fresh-water pond or stream (name) we take a little water 

 with some duckweed or other water- weeds, and allow them to stand in 

 a glass for some hours, we shall, if we are fortunate, easily discover 

 fresh-water polyps. 



B. Appearance and Species. 



They will be found attached to the side of the glass or other object 

 by one end of their cylindrical body, while at the other end from six to 

 twelve very fine hair-like threads, the so-called tentacles, play freely 

 about in the water. These polyps vary considerably in colour. Most 

 frequently they are brownish, less commonly green, and still more 

 rarely of grey or reddish colour. These differently-coloured examples, 

 however, display also regular differences in their finer structure (these do 

 not interest us here), and are therefore described as three distinct species, 

 named, according to their predominant tint, respectively the Brown, 

 Green,* and Grey Fresh -Water Polyps (H.fusca, viridis, and grisea). 

 These different forms also vary in size, for whilst the green hydra is 



* The green hydra derives its colour from unicellular algoe {ZoocMorella) embedded in its 

 tissue. These live upon the carbonic acid gas expired by the animal, to which on their part 

 they give up in exchange oxygen and probably food materials which they have elaborated. 

 We have here a case of combination for mutual benefit (symbiosis), between an animal and a 

 plant, which is repeated in several other (green-coloured) animals. 



