THE ANIMAL KINGDOM 87 



Protozoa number about 8500 living and 200 fossil 

 species. 



n. Porifera. The sponges, chiefly marine an- 

 imals composed of many cells surrounding pores and 

 channels through which water circulates freely. 

 Besides budding, they are reproduced by eggs and 

 sperms. The soft living "sponge-flesh" is usually 

 supported by spicules and, in different varieties, by 

 skeletons of horny, calcareous or silicious material. 

 The horny sponges are of commercial importance. 

 The calcareous sponges and glass sponges are fre- 

 quently found as fossils. Fresh water sponges, 

 usually inconspicuous, are widely distributed but of 

 no commercial value. Approximately 2 500 living and 

 800 fossil species are known. 



III. Coelenterata. A group of slightly complex 

 animals embracing the fresh water hydra and nu- 

 merous salt water forms, as the polyps, jelly-fish, 

 coral-forming animals, etc. They exhibit a radial 

 arrangement of parts and most of them possess 

 peculiar stinging cells. The first distinctly developed 

 nervous system and sense-organs appear in this 

 group. Four thousand two hundred living and nearly 

 2000 fossil species are recognized. 



The animals formerly grouped under the general 



