The American Spongilla. 109 



diameters it displays a very finely granular aspect. It occupies 

 wide intervals between the cells, certainly more than one-half, and 

 fully three-fifths of the whole area of the membrane. Its apparent 

 extent, in a general view, is even more than that, owing to the 

 extreme transparency of the cells, and their consequent incon- 

 spicuousness. That the cytoblastema, notwithstanding its low un- 

 developed state, is the true contractile element in this membrane, 

 there can scarcely be a doubt, when we consider both its wide- 

 spread preponderance and its relative continuity, as contrasted witli 

 the scattered, disconnected condition of the cells (h') which are im- 

 bedded in it. Sometimes it is barely possible to discover even the 

 trace of a cell on the border of an afferent ostiole (os), and in that 

 case we must infer, inevitably, that it is cytoblastema which opens 

 and closes the aperture. We find it, too, embracing the extreme 

 tips of the larger spiculae, where the cells utterly fail to appear. 



The cell element (b) of this membrane is also in a lowly con- 

 dition ; only partially developed. There is no cell wall. What 

 may appear to be a wall is really the thin stratum of cytoblastema 

 (a}) overlying the distal and proximal faces of the cell. This is 

 our conclusion after the most critical scrutiny, with a carefully- 

 corrected objective. Were it not, indeed, for the usually constant 

 presence of a distinct nucleus {n) in each cell, we would be strongly 

 inclined to look upon it as merely a dense collection of coarser 

 granules than are generally diffused through the cytoblastemic 

 layer. The irregular and jagged outline, and the caudate projec- 

 tions of the cells (b-), also tend to tempt one to tbe latter view. 

 The cell element in this case, then, corresponds only to what is 

 usually considered the cell contents, and a nucleus. The contents 

 are composed of coarse and fine grey granules, which at times are 

 quite conspicuous, but most frequently are so transparent and 

 slightly refractive as to appear, collectively, unless specially focussed 

 upon, as a faint blotch in the investing membrane. This renders 

 it all the more difficult to trace the outline of the cell, and par- 

 ticularly v/here it throws out irregular, caudate prolongations, to 

 blend with those of other cells. We have been able to detect but 

 one layer of cells in this membrane * when it is well stretched out. 

 The depth of the cells, as may be seen in a sectional profile view (b), 

 is about equal to their breadth, and their length is from one-half 

 more than to twice their breadth ; but frequently they are as broad 

 as long. . They stand in no particular relation to the ostioles ; and, 

 as stated above, sometimes scarcely touch their border. The nucleus 

 (n) may be readily detected by its peculiar, strong refraction, and its 

 considerable superiority in size over the granules. Its bright re- 

 fractiveness in this connection reminded us of a contractile vesicle, 



* Carter figures two or three cells overlying each other in Spomjilla alba, 'Ann. 

 Mag. Nat. Hist.,' July, 1857, PI. 1, fig. 7. 



