106 COLLEGE ZOOLOGY 



choanocytes of sponges recall the choanoflagellate Protozoa 

 (p. 47), and it is not improbable that they may have evolved 

 from this group. Certain colonial choanoflagellates, e.g. Protero- 

 spongia (Fig. 29) resemble what we might imagine to have been 

 the ancestor of the sponges. 



(6) The Relations of Sponges to Other Organisms and to Man. — 

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

 tected by spicules and by excretions of poisonous ferments mak- 

 ing them distasteful. Nudibranch mollusks (Chap. XII) feed 

 on them to a certain extent. The cavities of sponges offer shel- 

 ter to many animals, especially 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 which cover 

 their shells and take away their food supply, and oyster cultur- 

 ists often prevent this by growing the bivalves in frames which 

 are pulled up during a rain, thus killing the sponges with fresh 

 water. 



The origin of flint is in part due to the activities of sponges. 

 It has been estimated that to extract one ounce of silicious spicules 

 at least a ton of sea water must pass through the canal system 

 of the sponge. The spicules aid in the formation of flint, this 

 substance being always associated with the remains of sponges, 

 Radiolaria (p. 40), and other organisms having silicious skele- 

 tons. 



Of the commercial sponges may be mentioned the beautiful 

 skeleton of Venus' flower-basket, Euplectella (Fig. 62), which 

 is obtained chiefly in the Philippine Islands, and the common 

 bath sponge, Euspongia (Fig. 63) , and others, which are especially 

 grown for market in some localities. The best bath sponges 

 come from the Mediterranean coast, Australia, the Bahamas, 

 Florida, and the north coast of Cuba. They are gathered by 

 means of long hooks, by divers, or by dredging. They are al- 

 lowed to decay, are washed, dried, and then sent to market. 



