128 THE MECHANICAL FUNCTIONS. 



fixing their permanent abode, and proceeding in their far- 

 ther development. The tune of their remaining in this free 

 and moving state varies according to circumstances, 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 gemmules has be- 

 gun to fix, it be again disturbed, and separated from the 

 surface to which it had become attached, it generally re- 

 mains free, and perishes. During the process of fixing, it 

 exhibits 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 immediate- 

 ly surrounding the gemmule. 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 af- 

 ter disappear: then the gemmule begins to swell, the sur- 

 rounding margin becomes more transparent, and the whole 

 gradually assumes the form of a cell, surrounded by a deli- 

 cate white opaque line, which is the rudiment of the calca- 

 reous wall of the future cell. Towards the base of this ru- 

 dimental cell, the gelatinous substance in the interior may 

 be perceived to become more consistent and opaque at a 

 particular point; from this dull spot within the cell, short 

 straight tentacula begin to bud, extending upwards in the di- 

 rection of the future aperture. The gelatinous spot, from 

 which the tentacula originated, assumes the vermiform ap- 

 pearance of the body of a polype; and we may distinctly 

 perceive the bundles of fibres which connect its head with 

 the base of the cell. The structure of the polype is per- 

 fected by the addition of a closed capsule; and when it is 

 first detected protruding from the cell, it possesses all the 

 parts of an adult polype, and vibrates the cilia of its tenta- 

 cula vvith as much regularity and velocity as at any future 

 period. Before the polype is capable of protruding from 



