as a Craspedote Flagellate Infusorian, 75 



the cells and their consequent inconspicuousness. That the 

 cytoblastema, notwithstanding its low, undeveloped state, is the 

 true contractile element in this membrane there can scarcely 

 be a doubt, when we consider both its wide-spread preponder- 

 ance and its relative continuity, as contrasted with the scattered, 

 disconnected condition of the cells (J^) which are imbedded 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 spicula, where the cells utterly fail 

 to appear. 



The cell-element (h) 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 cytoblas- 

 tema (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 should 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 projections of the cells (Z>^) also tend to tempt one 

 to the latter view. The cell-element in this case, then, corre- 

 sponds 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 fre- 

 quently are so transparent and slightly refractive as to appear, 

 collectively, unless specially focused upon, as a faint blotch in 

 the investing membrane. This renders it all the more difficult 

 to trace the outline of the cell, and particularly where 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 (h), is 

 about equal to their breadth ; and their length is from one half 

 to more than 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 refractiveness in this connexion re- 

 minded us of a contractile vesicle ; but, although suspecting it of 

 such a function, Ave could detect no change other than might be 



* Carter figures two or three cells overlying each other in Spongilla 

 alba (Ann. & Mag. Nat. Hist., July 1857, pi. 1. fig. 7). 



