SPONGES. 121 



and some of them shoot into the thin homogeneous mnrcin. 

 It is a rcmarkahle circumstance that the spicula make their 

 appearance completely formed, as if by a sudden act of 

 crystallization, and never afterwards increase their dimen- 

 sions. 



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 interrup- 

 tion; they thicken and produce spicula: in a few days we 

 can detect no line of distinction between them, and they 

 continue 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 graft- 

 ing we again find an analogy between the constitution of 

 zoophytes and that of plants. 



In the course of a few weeks, the spicula are assembled 

 in groups, similar to those of the parent sponge; assuming 

 circular arrangements, and presenting distinct openings at 

 the points they enclose. The young animal now rapidly 

 spreads and enlarges in every direction, becoming more 

 convex, and at the same time more opaque, and more com- 

 pact in its texture; and before it has attained the tenth of an 

 inch in diameter, it presents, through the microscope, a mi- 

 niature representation of its parent. 



Thus has a power of spontaneous motion been given to 

 what may be regarded as the embryo condition of animals, 

 which are afterwards so remarkable for their inertness, and 

 for the privation of all active powers: and this has been con- 

 ferred evidently for the purpose of their being widely disse- 

 minated over the globe. Had not this apparatus of moving 

 cilia been provided to the gemmules of such species as 

 hang vertically from the roofs of caves, they would have 

 sunk to the bottom of the water and been crushed or buried 

 among the moving sand, instead of su])porting themselves 

 while carried to a distance by the waves and tides of the 

 ocean. Many species which abound in the Red Sea and 



Indian Ocean have, in this way, been gradually transported. 

 Vol. I. 16 



