170 THE MECHANICAL FUNCTIONS. 



circles, with no other apparent object than to 

 keep themselves afloat, until they shall arrive at 

 a favourable spot for fixing their permanent 

 abode, and proceeding in their further develope- 

 ment. The time of their remaining in this free 

 and moving state varies according to circum- 

 stances, from a few hours to about three days. 

 When about to fix, the slightest agitation of the 

 water causes them to desist, and to recommence 

 their gliding motions, which they continue for 

 some time longer. If, when any of these gem- 

 mules has begun to fix, it be again disturbed, 

 and separated from the surface to which it had 

 become attached, it generally remains free, and 

 perishes. During the process of fixing, it exhi- 

 bits no peculiar appearance or change of form ; 

 it simply lies on its side ; and the cilia continue 

 to vibrate over the whole surface, producing a 

 constant current in the water, apparently for the 

 purpose of cleaning the space immediately sur- 

 rounding the gem mule. It remains for three 

 days in this attitude, without undergoing any 

 perceptible change of form, and without relaxing 

 the vibrations of its cilia. At the end of this 

 time, the cilia cease to move, and shortly after 

 disappear : then the gemmule begins to swell, 

 the surrounding margin becomes more trans- 

 parent, and the whole gradually assumes the 

 form of a cell, surrounded by a delicate white 

 opaque line, which is the rudiment of the calca- 



