OF THE NATURE OF SPONGES. .51 



Dr Grant, on the contrary, ascertained that certain spe- 

 cies of sponges are entirely formed of an iinpatterned net- 

 work of corneous fibres, which he asserted were tubular 

 throughout ; that, in others, innumerous sharp spicula of 

 pure silex/ and of a crystalline appearance, are mixed uj) 

 with the horny material ; while in a third class the spicula 

 are composed of carbonate of lime alone. These spicula 

 of silex and lime he never found to coexist in the same 

 sponge ; and their figure is so various in the different spe- 

 cies, that they become valuable aids in enabling us to dis- 

 tinguish them. * 



The propagation of the sponges was another object of 

 inquiry. " When we cut," says Dr Grant, " a thin piece 

 off the surface of a living sponge, and look down through 

 one of its pores with the reflecting microscope, we perceive, 

 immediately beneath the projecting spicula which defend 

 the pore, a very delicate net-work of gelatinous threads 

 thrown over the entrance of the tube. This piece of struc- 

 ture is so fine, as to be perfectly invisible to the naked 

 eye : it consists of five or six threads, which pass in from 

 the sides of the tube to be connected with a central mesh, 

 so that there are six or seven meshes thus formed ; and 

 while this soft apparatus is beautifully defended by the 

 protecting spicula of the pore, it serves still farther to 

 guard the interior of the animal from the smallest particles 

 of sand, or the minutest visible animalcules. Along the 

 whole interior of the pores and tubes, there is a thin gelati- 

 nous matter enveloping every fibre, and filling all the in- 

 terstices between the fasciculi. This gelatinous matter is 

 transparent and colourless, and so little consistent, that it 



* Edin. Pliil. Joiini. xiv. p. 84-j ; ami ]). •■{■"in. 



0„ 0- tU V% LIBRARY 



