68 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



cells and food substance. These gemmules survive the 

 winter, and.in the spring the cellular contents come out 

 and develop into a sponge. 



The sponge individual contains one exhalant orifice 

 (osculum), with the channels leading into it. An ordi- 

 nary bathing sponge constitutes a colony of such indi- 

 viduals, which are not definitely marked off from each 

 other. Some other sponges have only one osculum, and 

 such are a single individual, e.g. Grantia. 



Excepting a few small fresh-water species (as Spon- 

 gilla\ sponges are marine. In the former, the cellular 

 part is greenish, containing chlorophyll ; in the latter, it 

 is brown, red, or purple. In preparing the sponge of 

 commerce, this is rotted by exposure, and washed out. 

 The best fishing grounds are the eastern end of the 

 Mediterranean and around the Bahama Islands. 



Branch III. CCELENTERATA 



In the animals comprising this group, the body 

 cavity is not distinctly separated from the digestive 

 cavity. The ccelenterates are almost wholly marine 

 forms, hydroids, corals, sea anemones and jellyfishes, 

 but there are a few which, like Hydra, live in fresh 

 water. The body is usually radially symmetrical and 

 shows three more or less definite cell layers, the ecto- 

 derm on the outer surface, the endoderm lining the inner 

 cavities, with the mesoderm, or middle layer, between 

 the others. In hydra and the hydroids the mesoderm 

 is reduced to a mere film, but in the jellyfishes and 

 sea anemones it forms a large part of the body. A 

 characteristic feature is the presence of the stinging 

 cells, or nematocysts, which are almost invariably to be 

 found except in one group, Ctenophora, where they 

 are replaced by adhesive cells. 



