562 NUTRITION AND GROWTH OP SPONGE. 



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 Leuconia 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. 



1143. 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 cut 

 with sharp instruments, pierced with red-hot wires, or torn into 

 fragments; and yet no change of form, or other evidence of sensi- 

 bility, can be perceived in them. The parts which are unimpaired 

 will continue to present the same phenomena as before ; and no 

 injury seems to have any influence, beyond the portion imme- 

 diately affected by it. Nevertheless some observers maintain that 

 a shock, affecting the whole mass alike, does produce an evident 

 effect upon it. This has been particularly noticed in the 

 Spongilla, or River-sponge. "When allowed to fall into water 

 from the height of a few inches, or when the stage of the 

 miscroscope is struck, the tubular prolongations on which the 

 vents are situated contract very sensibly, until the orifice is 

 nearly closed, and projects but slightly. It has also been 

 noticed that these projecting orifices vary considerably in their 

 form at different times, even within short intervals, and when 

 no external cause has influenced them. Some Naturalists state, 

 moreover, that, although no sensible contractions and dilatations 

 can be seen in the whole mass, a peculiar sensation is felt when 

 the hand is placed upon a specimen still under water. This 

 sensation is of a tingling character, and appears due to some 



