120 THE MECHANICAL FUNCTIONS. 



the water by a rapid succession of inflections, apparently 

 made without any regular order, but conspiring to give an 

 impulse in a particular direction. When the body is at- 

 tached by its tail, or narrow end, to some fixed object, the 

 motion of the cilia on the fore part of the body determines 

 a current of fluid to pass in a direction backwards, or to- 

 wards the tail; but when they are floating in the water, the 

 same action propels them forwards in the opposite direction, 

 that is, with the broad ciliated extremity foremost. They 

 thus advance, without appearing to have any definite object, 

 by a slow gliding motion, totally unlike the zig-zag course 

 of animalcules 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 vibrations, proceed 

 in their former course. 



In about two or three days after these gemmules have quit- 

 ted the body of the parent, they are observed to fix themselves 

 on the sides or bottom of the vessel in which they are con- 

 tained; and some of them are found spread out, like a thin cir- 

 cular membrane on the surface of the water. In the former 

 case, they adhere firmly by their narrow extremity, which is 

 seen gradually 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 certain 

 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 granules, like 

 the flesh of the parent sponge; and also several spiculae in- 

 terspersed through the central part. In less than twenty- 

 four hours, a transparent colourless margin has extended 

 round the whole gemmule, and continues to surround it 

 during its future growth. The spicula, which were at first 

 small, confined to the central part, and not exceeding twenty 

 in number, now become much larger and more numerous; 



