SPONGES 135 



through foraminal openings or tubules, or by rupture of the shells, and 

 develop into new sponges (Fig. 56). 



Sexual reproduction also occurs, both egg cells and sperm cells being 

 formed in the same animal. These sex cells lie in the jelly-like middle 

 layer where fertiUzation takes place. An embryo is formed which escapes 

 through the wall of the body and becomes a free-swimming ciUated larva. 

 This later settles down and develops into a sponge. 



159. Uses of Sponges. — The cleaned skeletons of those sponges which 

 are composed entirely of spongin are familiar because of their many 

 domestic uses. Among these are bath sponges and the surgeons' sponges 

 used to take up blood and other fluids in surgical operations. Though 

 today artificial sponges are made which in many cases take the place of 

 natural ones, there is still a large market for the latter. 



160. Cultivation of Sponges. — The best commercial fibrous sponges 

 come from the coast of the Mediterranean Sea, from the shores of Florida 

 and the West Indies, and from Australia. They are gathered by means 

 of long-handled hooks, by dredging, or by divers. They are then allowed 

 to decay, are washed, dried, bleached more or less, and sent to market. 



Sponge culture is now carried on in several localities but most suc- 

 cessfully in Italy and Florida. Commercial sponges do not flourish 

 where the water is cold. The place selected for this purpose must have 

 a clean bottom and must be exposed to currents which bring an abundant 

 supply of well-aerated water and food. Specimens of the variety of 

 sponge to be cultivated are secured, cut into small pieces approximately 

 one inch square, and fastened either to stakes or to sunken cement 

 plates. From these pieces grow complete sponges which are ready for 

 the market in a few years, the time depending upon the character of the 

 sponge grown and the conditions. When the sponge is gathered, the 

 part that remains after most of it is cut away will continue to grow and 

 develop into another sponge. 



161. Relations to Other Animals. — Sponges are used as food by very 

 few animals, their spicules and the unpleasant character of their excre- 

 tions rendering them objectionable. Because of this fact many other 

 animals take refuge in sponges. The excretions of sponges also play a 

 part in the disintegration of the empty shells of mollusks, the lime of 

 which is thus turned back into the sea water to be used over again by 

 other animals. 





