ANIMAli NATURE OF D.IATOMEiE. 361 



many genera, and especially in the AchnantJiidia, the 

 siliceous shield is covered with a very delicate dilatable 

 membrane, itself containing silica, as is proved by its 

 sustaining unchanged the action of fire and acids. There- 

 fore, comparing this shield with other organic formations, 

 whether animal or vegetable, containing, in like manner, 

 either silica or some other so-called mineral element, we 

 may reasonably consider it to be formed of an organic 

 tissue permeated by silica. This permeation may occur 

 either in the wall of a simple cell, as is seen in the 

 epidermal cells of many plants, or within minute cells, as 

 in various plants and animals. The action of heat or of 

 acid, in these cases, destroying the organic matter and 

 leaving the silica untouched, does not alter the ap- 

 parent form of the organ, because the skeleton remains 

 unaltered. 



Externally to the shield, Kiitzing observed a thin 

 stratum, which he denominated cement, which may be 

 made visible either by desiccation or by calcination, and 

 produces either a simple opacity, or lines, points, and 

 maculse, sometimes irregularly disposed, sometimes regu- 

 larly. He supposes it to be a silicate of iron or of alumina. 

 Independently of the chemical materials which it may 

 contain, this outside integument seems to me the more 

 important inasmuch as even without resorting to the 

 means indicated by Kiitzing, I observe it to be constant, 

 not merely in the species enumerated by him, but also 

 in many others, and I could almost assert that it exists 

 in all. For to me it appears to correspond with that fine 

 membrane of the AchnantJiidia above mentioned, which, 

 according to Kiitzing's own observations, is always 

 visible whenever the two new individuals, (into which 

 every Diatomean is resolved in its multiplication by de- 

 duplication) {sdoppiamento) begin to separate. The 

 lines and points supposed to belong to the subjacent 

 shield belong very frequently to this kind of covering. 



The shield itself is formed of at least four pieces, or 

 valves, united together in a four-sided figure — tetragon. 



