NUTRITION AND GROWTH OF SPONGE. 561 



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. According 

 to an observation of Mr. Bowerbank's, since confirmed by Mr. 

 Carter and other observers, the incessant passage of water 

 through the channels of Sponges, is to be attributed to the action 

 of cilia with which the cells lining the incurrent orifices are 

 lined. 



1227. 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. From these and from the water itself, 

 the animal tissue appears to derive the materials of its nourish- 

 ment ; and the silicious 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 

 exist in such minute proportion, as scarcely to be recognised by 

 the chemist. Not only does this circulation of fluid answer the 

 purposes of nutrition, but it is subservient also to the process of 

 excretion. On watching the currents of water that issue from 

 the vents, it is observed that minute flocculent particles are 

 incessantly detached and carried out by them ; these appear as 

 if they were the residue of digestion, or pellicles excreted from 

 the body and thrown off from the surface of the internal canals. 

 Sometimes the vents are all on one side, whilst the absorbent 

 pores are on the other ; and in one interesting species of a 

 bottle-like form, the Grantia compressa, the pores are entirely 

 situated on the outside, and the vents on the interior, so that a 

 single constant and rapid current of fluid proceeds from the 

 mouth, which is contracted into rather a narrow orifice. 



1228. This movement of fluid through their tissue seems 

 to be almost the only action that proves the existence of life in 

 these simple beings. No obvious contraction can be perceived 

 when they are touched or irritated. They may be punctured or 



