544 



SPONGES. 



Fresh- Water To the group under consideration belong the fresh- water sponges 



sponges. (Spongillidce), which live in ponds, canals, lakes, and rivers all over 

 the world ; and have been known to infest the pipes supplying a city with water. 

 The two commoner British species (Euspongilla lacustris and JEphydatiaflwviatilis) 

 grow on the piles of bridges, the sides of locks, the stems of water-weeds, or 

 form crusts on the bed of rivers. Euspongilla forms bright green crusts, from 

 the surface of which long, simple, or branched stems arise ; or the surface of the 

 crust may be simply conulated. This green colour is due to granular bodies which 

 crowd the cells near the surface of the sponge. Some naturalists consider these 

 bodies to be chlorophyll granules similar to those of plants ; others regard them as 

 single-celled algas. The chlorophyll, in the presence of sunlight and water, splits 



LIMESTONE BORED BY SPONGE. 



up the carbonic acid evolved by the sponge into carbon and oxygen, the latter 

 being used by the sponge for respiration. Fresh-water sponges growing in shady 

 places are of a pale grey or yellowish white colour; and when bright green 

 specimens are kept in the dark, they lose their green colour. The surface of 

 a fresh - water sponge is covered with fine pores, while here and there a few 

 large oscules are visible. From the pores fine in-current canals pass down to the 

 flagellated chambers, and from the latter proceed the rootlets of the out-current 

 canal-system. With a lens the spindle-shaped siliceous spicules of the skeleton can 

 be made out. They are about one-fiftieth of an inch in length and unite in bundles 

 which partly surround the canals, and are partly scattered irregularly in the ground 

 substance ; with the naked eye the bristling points can be seen projecting from 

 the surface. If a specimen be examined in autumn, there will generally be found 

 crowding the meshes at the base of the crust a number of small yellow spheres, 

 about one-twelfth of an inch in diameter, known as gemmules. They possess a firm 

 shell, with a small circular pore at one spot covered only by thin membrane. A 

 gemmule is a kind of internal bud, and is capable of developing into a new sponge. 



