114 NOURISHMENT OF SPONGES. 



sited, and from it the whole growth of the mass pro- 

 ceeds. The slimy substance is apparently void of sen- 

 sation, for it does not shrink when wounded ; and the 

 only motion resembling animal life which the mature 

 Sponge exhibits is in the imbibition and expulsion of 

 continuous currents of water. If any species of Sponge 

 be examined, the holes with which the substance is 

 everywhere pierced may be seen to be of two kinds, one 

 of larger size than the rest, few in number, and opening 

 into wide channels, or tunnels, which pierce the Sponge 

 through its centre j the other minute, extremely nume- 

 rous, covering the whole surface, and communicating 

 with the innumerable branching passages which make 

 up the body of the skeleton. According to the obser- 

 vations of Dr. Grant, water is freely imbibed through 

 the smaller holes, and continuously expelled in jets 

 through the larger, as long as the animal retains life. 

 These currents may be seen if a small specimen of a 

 living Sponge be placed in a watch-glass or other shallow 

 vessel of salt-water, and examined through the micro- 

 scope ; and it appears to be through their agency that 

 the substance is nourished. Nourishing particles dis- 

 persed through the water are received into the universal 

 stomach, and what is not required is ejected through 

 the canals. 



Such is the simple history of the Sponges. Their pro- 

 pagation is provided for in a curious manner. At cer- 

 tain seasons of the year, if a Sponge be cut open, innu-. 

 merable minute bud-like points will be found attached 

 to the sides of the lining of the canals. These are the 

 gemmules or young eggs of the sponge. As they increase 



