SPONGES. 159 



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 spi- 

 cula, which were at first small, confined to the 

 central part, and not exceeding twenty in num- 

 ber, now become much larger and more nume- 

 rous ; and some of them shoot into the thin ho- 

 mogeneous margin. It is a remarkable circum- 

 stance that the spicula make their appearance 

 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 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 

 grafting 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 



