144 THE MECHANICAL FUNCTIONS. 



While this is going on, the ciHa 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 spicula interspersed 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 exceed- 

 ing twenty in number, now become much larger 

 and more numerous ; and some of them shoot into 

 the thin homogeneous margin. It is a remarkable 

 circumstance that the spicula make their appear- 

 ance completely formed, as if by a sudden act of 

 crystallization, and never afterwards increase their 

 dimensions. 



When two gemmules, in the course of their 

 spreading on the surface of a watch-glass, come 

 into contact with each other, their clear margins 

 unite without the least interruption ; they thicken 

 and produce spicula : in a few days we can detect 

 no line of distinction between them, and they con- 

 tinue to grow as one animal. The same thing 

 happens, according to the observation of Cavolini, 

 to adult sponges, which, on coming into mutual 

 contact, grow together and form an inseparable 

 union. In this species of animal grafting we again 



