154 THE MECHANICAL FUNCTIONS. 



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 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 clearing the 

 space immediately surrounding the gemmule. It 

 remains for three days in this attitude, without un- 

 dergoing any perceptible change of form, and with- 

 out 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 transparent, 

 and the whole gradually assumes the form of a cell, 

 surrounded by a delicate white opaque line, which 

 is the rudiment of the calcareous wall of the future 

 cell. Towards the base of this rudimental cell, the 

 gelatinous substance in the interior may be per- 

 ceived to become more consistent and opaque at a 

 particular point ; from this dull spot within the 

 cell, short straight tentacula begin to bud, extend- 

 ing upwards in the direction of the future aperture. 

 The gelatinous spot, from which the tentacula ori- 

 ginated, assumes the vermiform appearance 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 



