170 Dr Grant on the Structure of some Calcareous Sponges. 



species, presented me by Dr Fleming, under the name of 

 S. pulverulenta, presents two kinds of spicula, both of which 

 effervesce and dissolve quickly in nitric acid ; one of these 

 forms is a triradiate spiculum with long and very slender rays 

 diverging at equal angles ; the other is a very long straight 

 needle-shaped spiculum, pointed acutely at one end, and ob- 

 tuse at the other. This calcareous species agrees with the 

 others in its white colour, and the silvery lustre of its spicula, 

 when dry. The Spongia coronata is the most minute and the 

 most perfectly constructed of all the calcareous sponges I have 

 yet met with. It has two kinds of spicula, the one triradiate, 

 and the other needle-shaped, both of which dissolve quickly 

 with eifervescence in diluted acids. The triradiate spicula are 

 more equal in size than in the other species, and are models of 

 this form for their symmetry and proportions ; the rays are 

 straight, slender, and diverge equally; they are cylindrical, 

 transparent, and acutely pointed. The needle-shaped spicula are 

 about twice as long as the triradiate, slender, transparent, cylin- 

 drical, rounded at one end, and pointed acutely at the other. 

 This sponge is almost microscopic ; several entire specimens of 

 it, presented me by Dr Fleming, are not half a line in length ; 

 they agree with the others in their colour, and the lustre of 

 their spicula. The long needle-shaped spicula cover the whole 

 surface, like filaments of white silk, and are obviously destined 

 to defend the pores and the fecal orifice, which is proportionably 

 large. On removing these projecting needle-shaped spicula 

 from the surface, which may be compared with the clavate spi- 

 cula of the S. compressa, we observe that the triradiate spicula 

 are entirely devoted to the formation of the pores and passages 

 leading into this animated tube. 



There are thus at least six well marked species of British 

 sponge, in which the spicula consist entirely of carbonate of lime, 

 which forms an important character of distinction between these 

 species, and those containing a horny or a siliceous axis, and 

 shows an approximation in this obscure genus to the more 

 solid polypiferous corals, which, so far as I know, has hitherto 

 escaped notice. 



