INTRODUCTION. 



Section III. 



Internal Structure and Cell-contents of the Diato- 



MACEOUS FrUSTULE. 



I now proceed to describe the living frustule, some of the 

 appearances presented by which can only be understood by a 

 previous acquaintance with the structui'al character derived from 

 an examination of the siliceous envelope. 



A careful study of these organisms in a fresh state is neces- 

 sary to enable the observer to arrive at a correct idea of their 

 true character. The species are liable to material alterations 

 when removed for any length of time from their natural localities ; 

 the lapse of a few days, and sometimes of a few hours, being 

 sufficient to modify or altogether change the colour and arrange- 

 ment of the cell-contents, and to suspend the peculiar movements 

 which in many species accompany their living condition. They 

 must, therefore, be examined under favourable circumstances, 

 or om- conclusions may be of the most erroneous kind ; and a 

 neglect of these precautions has undoubtedly led in some cases 

 to grave errors respecting the structm'c, and even the nature, of 

 the Diatomaceous frustule. As I look upon these organisms as 

 unicellular plants, I consequently regard the siliceous valves as 

 investing a closed membranous sac or cell. Nor is this a matter 

 of conjectiu-e and analogy only. In some cases I have seen 

 unmistakeable indications of the existence of this membrane, 

 which I have noticed to contract around the cell-contents upon 

 the death of the cell; and in one species, Staiironeis pulcheUa, 

 Plate XIX. fig. 194, in which the membrane in question appears 

 to possess an unusual degree of firmness, the siliceous valves, after 

 a slight maceration in acid, may be seen to fall away from the 

 internal membrane, and to leave the latter unaltered in form. 

 Moreover, the experiments and authority of Professor Bailey 

 place the existence of an internal membrane beyond all doubt. 

 This acute observer states that, " If hydrofluoric acid is applied 

 to recent Diatomacese, the shell soon dissolves, leaving distinct 

 internal, flexible cell-membranes, retaining the general form of 



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