THE SPONGES 215 



Spicules and Spongifl. The body wall of most sponges is 

 supported by spicules of calcium carbonate or of silica (Fig. 

 129, A), and a few like the bath sponge have a skeleton of fibers 

 consisting of a substance called spongin (Fig. 129, B). 



The Relations of Sponges to Other Organisms and to Man. 

 Sponges are used as food by very few animals, since they are 

 protected by spicules and by excretions of poisonous ferments, 



FIG. 130. Looking for sponges through a glass-bottom pail. (From Bui. U. S. 



Fish Com.) 



making them distasteful. Nudibranch mollusks (see p. 164) 

 feed on them to a certain extent. 



The cavities of sponges offer shelter to many animals, es- 

 pecially Crustacea and ccelenterates; this may lead to a sort of 

 partnership called commensalism. For example, certain hermit 

 crabs protect themselves from attack by surrounding their 

 shells with obnoxious sponges. 



Oysters and other bivalves are often starved by sponges 



