THE SPONGES 



215 



Spicules and Spongin. — 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 



