210 COMPARATIVE ZOOLOGY. 



A few Sponges, as the Yenus's Flower -basket (Euplec- 

 telld), have siliceous and others have calcareous skeletons. 

 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 Medi- 

 terranean and around the Bahama Islands. 



Subkingdom II. CCELENTERATA. 

 These radiate animals are distinguished by having a dis- 

 tinct cavity, whose walls have, at least, two layers of cel- 

 lular tissue, an outer (ectoderm) and inner (endoderm), and 

 usually a middle layer (mesoderm). They have thread cells, 

 minute sacs containing a fluid, and connected with barbed 

 filaments capable of being thrown out for stinging pur- 

 poses. Most are provided with hollow tentacles around 

 the mouth. All are aquatic, and nearly all are marine. 

 There are two classes, represented by the Hydra and Sea- 

 anemone. Both reproduce by budding and by eggs. 



CLASS I. Hydrozoa. 



These Coelenterates have no separate digestive sac, so 

 that the body is a simple tube, or cavity, into which the 

 mouth opens. The nervous system is slightly developed. 

 Such are the fresh-water Hydra and the oceanic Jelly-fish 

 (Acaleph or Medusa). 



The body of the Hydra is tubular, soft, and sensitive, 

 of a greenish or reddish color, and seldom over half an 

 inch long. It is found spontaneously attached by one 

 end to submerged plants, while the free end contains the 

 orifice, or mouth, crowned with tentacles, by which the 

 creature feeds and creeps. The body-wall consists of two 

 cellular layers ectoderm and endoderm. These surround 



