280 Animal Biology 



sponges live in salt waters of the ocean (marine). They are attached to rocks 

 and other submerged objects. Many species contain gray, red, green, or brown 

 pigments. The type of skeleton of the sponge depends upon the species. The 

 following kinds of skeletons are common: (1) spicules of silicon (siliceous), 

 (2) spicules of calcium carbonate (calcareous), (3) fibers of spongin (horny). 



Fig. 80. — A number of Vorticella (class Infusoria) shown attached to an object 

 by means of the contractile stalk. (Copyright by General Biological Supply House, 

 Inc., Chicago.) 



The various classes into which the phylum is divided are determined by the type 

 of skeleton. The body wall of sponges is diploblastic (two layers of cells) ; a non- 

 cellular middle layer is known as the mesenchyme (mesoglea). The external 

 cellular layer is known as the ectoderm; the internal cellular layer, as the ento- 

 derm. Number of species of Porifera, 3,000. 



