56 INTRODUCTION TO ZOOLOGY CHAP. 



These sponges all live in deep water, whence they are 

 obtained by divers, or by means of long-handled pronged 

 forks with which they are speared from boats. The living 

 sponges are covered and penetrated throughout by the slimy 

 living tissues, and it is only after these have decayed and 

 the horny skeleton has been washed free from them, that the 

 sponge is ready for household use. 



There are two British fresh-water sponges, both 

 ^ wn i cn are peculiar in developing abundance of 

 chlorophyll l if growing in a light spot ; if in the 

 shade they are a buff colour. 



Ephydatia ( = Spongilla) fluviatilis is often very abundant in 

 rivers and canals, forming massive, dull green or yellow, slimy 

 encrustations on any old submerged timber, or growing on 

 the banks ; sometimes its surface is produced into a number 

 of short leaf-like lobes. In the autumn it reproduces freely 

 by means of gemmules (see below). 



Spongilla lacustris, the Pond Sponge, also forms encrusting 



masses, but from these grow out oval or finger-like processes. 



It is a brighter green than the River Sponge, and its substance 



is denser and is made of coarser fibres. 



Reproduc- Since these big complex sponges are formed by 



tion in the branching of a simple sac-like form, they may 

 Sponges. j^ I 00 k ec i upon as formed of a colony of sponge 

 individuals, living together in a united mass. Sometimes a 

 little group of cells will become detached from such a sponge 

 as a bud or gemmule, which will develop into a new separate 

 individual ; this is specially frequent in the common green 

 fresh-water sponge, Spongilla, where each gemmule is about 

 the size of a pin's head, and is surrounded by a thick horny 

 capsule strengthened by siliceous spicules. Inside this cap- 

 sule, the gemmule persists through the winter, giving jise to 

 a new sponge in the spring. If one such gemmule is re- 

 moved from the parent, arid kept in a covered watch-glass 

 of water in a sunny spot, after a few days an active young 

 sponge will be obtained, which is so transparent that the 

 currents of water passing through its body can be readily 

 traced if the water is coloured with carmine. 



Sponges also multiply by a sexual process similar to that 

 in Hydra, i.e. as a result of the fertilisation of an egg cell by a 

 1 See footnote, p. 28. 



