XIV INTRODUCTION. 



maintains a connection with a foreign body ; we have this form 

 in Synedra and Gomphonema, &c. In others, again, it is enclosed, 

 with multitudes of its confreres, in a gelatinous or membrana- 

 ceous thalhis, as in Schizonema, Encyonema and their allies, and 

 the aggregation of the frustules in such cases sometimes rivals 

 in form and size the branching filaments of the larger Algae, 



Upon submitting living DiatomaccEe to a microscopic exami- 

 nation, theii' forms are found to be exceedingly diversified, though 

 the mineral nature of their epiderm confers upon them a general 

 character which reminds the observer of inorganic rather than of 

 vegetable or animal productions. Thus prismatic and quadri- 

 lateral forms are among the most general ; linear, crescent, or 

 boat-shaped outlines are not unfrequent ; while indidivuals grace- 

 fully curved in a sigmoid manner, and others resembhng minia- 

 ture shields or circular discs, occasionally present themselves. 



Section IL 



Structure of the Siliceous Epiderm of the Diatomaceous 



Frustule. 



To ascertain the tnie form and structure of the Diatomaceous 

 frustule requires a careful microscopic examination, not only 

 of living, but of desiccated or prepared specimens. Indeed it 

 is only after an examination of the latter, that the true character 

 of the living frustule can be understood. I shall therefore 

 commence my description of the structure of these organisms 

 by a reference to frustules which have been deprived of their 

 softer parts, either by submitting them to the action of fire, or 

 to the corrosive influence of an acid. 



The epiderm of the Diatom consists of two siliceous plates or 

 valves, usually of the most perfect symmetry. Wlien first pro- 

 duced, these valves are closely applied to each other, and the line 

 of junction forms a suture along which the valves readily sepa- 

 rate dm^ing the process of self-division w^hich speedily follows the 

 perfect formation of the cell. It seems to be a law with these 

 organisms, that no portion of the internal cell-membrane can be 



