286 



GENERAL ZOOLOGY 



growing in the sponge. Besides the species here described, 

 there are numerous other species of sponges living in 

 fresh water. 



In addition to sexual reproduction, fresh-water sponges 

 reproduce also by means of special organs called gem- 

 mules, which are about one fiftieth of an inch in diameter 

 (Fig. 148). These structures are produced in the sponge 

 mass during the summer. Their use is to carry the spe- 

 cies over the unfavorable season after the destruction of 



the parent sponge, 

 which usually takes 

 placeinthefall. The 

 gemmules are really 

 buds within the tis- 

 sue of the sponge 

 which become sur- 

 rounded by a pro- 

 tective covering. 



Relation to Envi- 

 ronment. Perhaps 

 no group of animals 

 has so wide a distribution in water as sponges. In fresh water, 

 and in the sea from the very margin of low tide to the great- 

 est depth of ocean yet explored, and in all zones, various 

 species of sponges are found. They live in every conceiv- 

 able situation, adapting themselves in form of mass to the 

 particular place in which they grow. Branched species, like 

 Microci'ona prolif'era, in the frontispiece, vary much in 

 form and arrangement of branch. Incrusting species on 

 rocks, as in the frontispiece, follow every turn of the sup- 

 porting substance. One species of Clio'na grows on shells 

 of mollusks, and through the agency of a secretion its pro- 

 toplasm consumes the substance of the shell and grows to 

 fill the space thus made. In regions where the ocean bottom 



Section of fresh-water sponge. 

 (Much enlarged) 



a, surface ; b, dermal pores ; c, ampullae, lined with 

 collar cells ; d, osculum. (After Huxley) 



