158 THE MECHANICAL FUNCTIONS. 



most. They thus advance, without appearing to 

 have any definite object, by a slow gliding 

 motion, totally unlike the zig-zag course of ani- 

 malcules in search of prey. Yet they appear to 

 have a consciousness of impressions made on 

 them ; for on striking against each other, or 

 meeting any obstacle, they retard a little the 

 motion of their cilia, wheel for a few seconds 

 round the spot, and then, renewing the vibra- 

 tions, proceed in their former course. 



In about two or three days after these gem- 

 mules have quitted the body of the parent, they 

 are observed to fix themselves on the sides or 

 bottom of the vessel in which they are contained ; 

 and some of them are found spread out, like a 

 thin circular membrane, on the surface of the 

 water. In the former case, they adhere firmly 

 by their narrow extremity, which is seen gra- 

 dually to expand itself laterally, so as to form a 

 broad base of attachment. While this is going 

 on, the cilia are still kept in rapid motion on 

 the upper part, scattering the opaque particles, 

 which may happen to be in the fluid, to a cer- 

 tain distance around. But these motions soon 

 become languid, and, in the course of a few hours, 

 cease ; and the cilia, being no longer wanted, 

 disappear. The gemmule then presents the 

 appearance of a flattened disk, containing gra- 

 nules, like the flesh of the parent sponge ; and 



