560 STRUCTURE AND ACTIONS OF SPONGE. 



meet with in the Vegetahje Kingdom ; and it is to their elas- 

 ticity that the Sponge owes those properties which make it so 

 useful to. Man. 



1225. In other forms of the group, however, spicula of earthy 

 matter are disposed amongst these ; which add to the firmness of 

 the structure, and diminish its elasticity, according to the amount 

 in which they are present. These spicula are in some instances 

 composed of carbonate of lime ; in others they consist of pure 

 silex ; and their form varies according to the material. The 

 earthy spicula are disposed at intervals through the whole mass ; 

 but they are especially abundant in the neighbourhood of the 

 canals, and around the external orifices, both of these and of the 

 pores. In fact each of these orifices is strengthened by a regu- 

 lar frame-work of spicula disposed around it, and in many in- 

 stances the sharp ends of the spicula project from the surface ot 

 the Sponge, as if for its defence. 



1226. When Sponges are examined in their living state and 

 natural condition, a constant and rapid stream of water is seen 



FIG. 730. SECTION OP LIVING SPONGE. 



to issue from the larger orifices or vents. This stream is made 

 apparent by the movement of the minute particles contained in 

 it, and by the disturbance of those which may be floating in the 

 surrounding fluid. On the other hand it is easily made apparent, 

 that water is as constantly being imbibed through the minute 

 pores ; and that, after traversing the smaller cavities of the 



