50 OPINIONS AND DISCOVERIES 



the forces enniloyed to produce the currents in any parti- 

 cular specimen." * — The power by which the currents are 

 set and kept in motion, and their circuit regulated, has not 

 been ascertained. Dr Grant could discover vibratile cilia 

 neither around the orifices nor on the walls of the canals 

 and pores, such as he discovered to originate similar cur- 

 rents in the polypiferous zoophytes ; but the analogousness 

 of the phenomena in the two orders of living beings, almost 

 forces a conjecture that they result from, and depend on, 

 the same cause, f 



To Dr Grant natm*alists are also indebted for the disco- 

 very of the various composition of the skeleton or frame- 

 work of the sponges. Donati had long previously de- 

 scribed and delineated certain crystalline spicula of which 

 some Adriatic species almost entirely consist ; | and Ellis 

 had sho\m their arrangement in a native sponge, § and was 

 fully aware of their more general prevalence and import- 

 ance ; 11 but little precise was known in relation to them. IF 



• Edin. Phil. Journ. xiii.p. 104-5 Dr Grant saw the currents, with 



the naked eye, also in Sp. cristata and compressa. Ibid. p. 103. 



f Brewster's Edin. Encyelop. xviii- p. 844. Edin. New Phil. Journ. 

 ii. p. 126-7. See also Sharpey in Cyclop. Anat. and Phys. i. p. 612. 



\ Nat. Mar. dell' Adriatioo. tab. 8 and 9. Venezia, 1 750. 



§ Eng. Corall. p. 81, pi. 16, fig. D 1. 



II " Some (sponges) being composed wholly of interwoven reticulated 

 fibres when others are composed of little masses of strait fibres of dif- 

 ferent sizes, from the most minute spicula to strong elastic shining spines 

 like small needles of one-third of an inch long ; besides these, there is 

 an intermediate sort between the reticulated and the finer fasciculated 

 kinds, which seem to partake of both sorts." Soland. Zooph. p. 182. 



f So little that even Hatchett, after an analysis of many species, as- 

 serts that he found them all similar in their composition. " They consist 

 of gelatine, which they gradually give out to water, and a thin brittle 

 membranous substance, which possesses the properties of cofigulable 

 albumen." Thomson's System of Chemistry, v. p. 564. Edin 1807. 



