246 COMPARATIVE ZOOLOGY. 



A few Sponges, as the Yenus's Flower -basket {Eupleo- 

 tella), 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 ont. 

 The best fishing-grounds are the eastern end of the Medi- 

 terranean and around the Bahama Islands. 



Subkingdom III.^-Ccelenteeata. 



These radiate animals are distinguished by iiaving a body 

 cavity, whose walls have, at least, two layers of cellular 

 tissue, an outer {ectoderm) and inner {endoderm), and usual- 

 ly a middle layer {mesoderm), this cavity serving for both 

 digestion and circulation. They have thread-cells, minute 

 sacs containing a fluid, and connected with barbed fila- 

 ments capable of being thrown ont for stinging purposes. 

 Most are provided with hollow tentacles around the mouth. 

 All are aquatic, and nearly all are marine. There are three 

 classes, represented by the Hydra, Sea-anemone, and Cte- 

 nophores. All reproduce by eggs, and the first two also 

 by budding. 



Class I. — Hydrozoa. 



These Coslenterates 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 ^y^Zra and the oceanic Jelly-fish 

 {Acaleph or Medusa). 



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

 of a greenish or brownish 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 



