44 OPINIONS AND DISCOVERIES 



ference, the splcula will be found to be disposed and 

 slightly united into fibres, leaving cells ; but when it is 

 broke transversely it appears to be formed irregularly ; 

 and in the Tethya the spicula ai'e very large, and are 

 felted together, with their points towards the surface, into 

 a hollow subglobular form, so that it is in fact originally a 

 tubercle, formed of one series of spicula, which from the 

 intervention of new spicula between the old ones, increases 

 in diameter, and thus produces the central space. The 

 spicula of species which I examined had the base slightly 

 cut into two or three radiating processes." 



Within a few months of their publication the opinions 

 of Mr Gray were controverted by IMr Thomas Bell, who 

 fully agreed that the fluviatile production that formed the 

 subject of Mr Gray's experiment was a true vegetable, 

 while he denied that the conclusion could be extended to 

 those species of sponges which are on all sides allowed to 

 belong to the group. " No longer ago than last autumn," 

 says Mr Bell, " being on the Dorsetshire coast, I ex- 

 amined the sti'ucture of recent marine sponge \\'ith care, 

 and certainly discovered no traces of the granules described 

 by Mr Gray, (and which, in fact, already formed one of 

 the generic characters of Spong'dla, according to Lamarck.) 

 One very important fact however I have ascertained, and 

 that is the power of contraction and dilatation in the tubes 

 of which this substance is composed. I placed a piece of 

 sponge, immediately after it was taken from the sea, in a 

 glass of sea-water ; a little dust had settled upon the sur- 

 face, and upon looking attentively at the water, the regu- 

 lar alternate motion and rest of the dust upon the surface, 

 immediately o\ex the mouths of the tubes, most distinctly 



