132 METAZOAN PHYLA 



animals and man, such as the muscle which shuts off the stomach from 

 the intestine, that which guards the exit from the bladder, and that 

 which controls the passage of egested matter from the posterior end of the 

 alimentary canal.) 



156. Reproduction. — Reproduction is both sexual and asexual. The 

 asexual mode of reproduction involves the gradual formation of buds 

 which arise near the point of attachment of the parent. After growing 

 for a time thus attached a bud may separate and begin an individual 

 existence. If budding continues and the individuals remain together, 

 a colony is produced (Fig. 50 ^). 



In addition to budding, some sponges have the ability to form 

 gemmules. These are groups of cells which gather together in the middle 

 layer and become surrounded by a siliceous shell. They are formed when 

 living conditions become difficult and thus preserve the life of the organ- 

 ism during such periods. In the fresh-water sponges gemmules are 

 formed in the autumn, after which the adults die, and in the spring the 



gemmules develop into new sponges (Fig. 53). 



^^i^fS^^^y:^ Sexual reproduction also occurs, both egg cells 



^!^^'^<>:^: M -ir::^^ and sperm cells being formed in the same animal. 



'Hi^^^^''^'^ki^^Mk '^^^^^ ^^^ ^^^^^ ^^^ ^"^ ^^^ jelly-like middle layer 



'Wi^-^W-^^^^ where fertilization takes place. An embryo is 



i^:ty<C(^:!'0'h'^^4m f*^^''"^6<^l which escapes through the wall of the body 



X*t%V^t*'44r/^ %^ *'^ ^i^<^ becomes a free-swimming ciliated larva. This 



N^^^S-;ft^0^ later settles down and develops into a sponge. 



Yia 53 —A gemmuie ^^'^ • Uses of SpoHges. — The cleaned skeletons 



of a fresh-water sponge of those sponges which are composed entirely of 



{hinn&eus) {FrVm spongin are familiar because of their many domes- 



Potts, Proc. Acad. Nat. tic uscs. Among these are bath sponges and the 



The rem^Selrcovered s^rgcons' sponges used to take up blood and 



by a chitinous shell in other fluids in surgical operations. Though today 



ulti!^Tlbout'?oo.'''''" artificial sponges are made which in many cases 



take the place of natural ones, there is still a 

 large market for the latter. 



158. 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 



