known Species o/" Spongilla. 83 



appearance like that of vegetable cell -structure when compressed 

 witVms any granules (fig. 12,/'). (For further detail respecting 

 these parts, see Nos. 12, 17, and 19.) Outside this follows a 

 comparatively thick, translucent, homogeneous membrane, seem- 

 ingly composed of chitine, whose amber colour being reflected 

 through the " axial" substance gives the latter in the section 

 a deeper tint than it is found to possess when separate ; this 

 will be called the " chitinous coat" (fig. 1, c). Then comes 

 another kind of coat, composed, in two instances, of cell-struc- 

 ture, which is almost evident under a doublet (fig. 3, (/and /), as 

 will be seen hereafter, but in the rest of a white granular 

 substance (fig. 2, a) that will presently be more particularly 

 described, which can only be resolved into such by a very 

 high compound power; and this will be called the " crust " 

 (fig. 1, d) ] it appears to aftbrd a floating property (like cork) 

 to the statoblast, but varies very much in thickness, not only 

 according to the amount of its development, but according to 

 the species. The " crust," again, is charged with, or accom- 

 panied by, minute spicules of different forms, variously ar- 

 ranged according to the species, which will be found by-and- 

 by, as before stated, to yield the chief characters of our classi- 

 fication (fig. 1, g, and fig. 2, h, c). At the " aperture," of 

 course, these two coats are deficient, while the interior or chiti- 

 nous one is prolonged into it by a tubular extension, generally 

 in proportion to the thickness of the " crust" (fig. 1, h). 



Meyen thought that the substance of the " crust " was 

 composed of " carbonate of lime having a cellular structure " 

 (No. 10, p. 154) ; but in no instance have I found it to effer- 

 vesce with acids, while, on the contrary, after boiling it for 

 some time in strong nitric acid it leaves a floccular residue, 

 which may be assumed to be a colloid form of silica, unless 

 it be undissolved tissue. As before stated, in some in- 

 stances the cell-structure, being comparatively large, is per- 

 fectly evident, while in others it is only resolvable under a 

 very high magnifying-power (at least 450 diameters), when 

 it may be termed " microcell-structure," presenting under 

 ordinary circumstances a white granular appearance, which, 

 filling up the intervals between the spicules, imparts to the 

 fully developed statoblast the light colour before mentioned. 

 It floats in water, and is very much like " pith," without 

 apparent cell-structure, is unaffected by liquor potassge, and 

 untinged by iodine, while before the blowpipe it burns off 

 without leaving any perceptible residue. The floating-power 

 of this substance is very considerable ; for it keeps on the 

 surface the whole of the internal contents, which swell out 

 and sink to the bottom the moment they are liberated by sec- 



