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 lacustr is and Epliydaticijiuvicitilis') 
grow on the piles of bridges, the sides of locks, the stems of water-weeds, or 
form crusts on the bed of rivers. Euspongillo, 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 algse. 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 w r hen bright green 
specimens are kept in the dark, they lose their green colour. The surface of 
a fresh - water sponge is covered with line pores, while here and there a few 
large oscules are visible. From the pores line 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, knou A 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. 
