Naoiculacea.] infusorial animalcules. 205 



Its bro^\^l coloiu' he would attribute to the probable presence of 

 iron in its chemical composition. 



Two views have been advanced relative to the origin of the ex- 

 ternal siliceous lamina ; one, that the silex exists in intimate union 

 with the cell, whose wall is believed to consist of cellulose pcneti-ated 

 with silica ; the other, that the siliceous valves are deposited 

 exterior to a cell-membrane. (Smith, Ann. Nat. Hist. 1851.) Nageli 

 says, " This es outside the membrane, and must be regarded, from 

 analogy to all other similar structures, as extra cellular substance 

 excreted from the cell" (Eay Society, 1846, p. 220.) But apart fi'om 

 analogy, Mr. Smith states he has direct evidence of the independence 

 of the siliceous coat, having in his possession numerous specimens of 

 A. Stauroneis (probably of S. asiyera, Kiitz.), in which the valves, 

 after a slight maceration of the frustulcs in acid, have, in part or 

 Avholly, become detached from the cell-membrane, leaving a scar on 

 its walls, bearing the distinct impression of the numerous and pro- 

 minent valvular marking of this beautiful species." StiU more 

 recentlj'. Prof. Bailey, of New York, states, that if hydrofluoric 

 acid is ajjplied to recent, or even sometimes to fossil JDiatomacea, the 

 sheU soon dissolves, leaving distinct internal, flexible cell membranes, 

 retaining the general foim of the shells. 



This second view of the separate existence of the inorganic sili- 

 ceous tunic, and its origin fi'om the organic internal membrane, 

 appears therefore the true one, from the preceding facts, and it is 

 still further supported by the phenomena of self-di-s-ision of the 

 fnistules; for in this process the lining cell-membrane takes the 

 initiative, and is followed by the doubling of the external coat upon 

 it, as an after production. 



Ehrenberg states the lorica to be composed of two or more pieces 

 or valves; Mr. Ealfs saj's, "it consists of three pieces, one central 

 and ring-like, or continuous all round, and the other lateral;" but 

 the investigations of the Eev. W. Smith would go to prove, that, in 

 not a few instances, the central ring-like segment is not an essential 

 one, but only developed when self-division is proceeding. In some 

 of the anomalous genera of Ehi-enberg, as jDictijocha, the individual 

 would seem to be composed of several pieces, varying in number in 

 various species. 



