NUTRITION AND GROWTH OF SPONGE. 561 



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

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



FIG. 629. SECTION OF LIVING SPONGE. 



that water is as constantly being imbibed through the minute 

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

 spongy structure, it finds its way into the canals through which 

 it is expelled. Some such absorption must take place, to supply 

 the fluid incessantly discharged through the vents. It is im- 

 possible to assign a cause for this movement ; no cilia have been 

 discovered in any part of the adult animal ; and the tissues are 

 altogether possessed of so little contractility, that it is difficult to 

 suppose the fluid propelled through the tubes by any mechanical 

 influence on their part. 



1142. The nutrition and growth of the Sponges appear to 

 depend entirely, then, on the water which enters the* pores, on 

 the substances which it holds in solution, and on the minute 

 particles suspended in it. The entrance of any but the smallest 

 solid substances is prevented by the network, which has been 

 described as stretching over the pores. From these, and from 

 the water itself, the animal tissue appears to derive the materials 

 of its nourishment ; and the siliceous and calcareous substances 

 must be separated by it, from the state of solution in which they 

 exist in the surrounding fluid. When the large quantity of this 

 fluid, which passes through the canals of the Sponge in a short 

 time, is taken into consideration, it is not difficult to account for 

 the derivation of these ingredients from it ; although they often 



