298 



THE MICROSCOPE AND ITS REVELATIONS. 



FIG. 137. 



marginal than central, as we see in Cocconema (Fig. 192). The Gompho- 

 nemece, like the Meridiece and Licmophorecu, have frustules which are 

 cuneate or wedge-shaped in their front view (Figs. 186, 187), but are dis- 

 tinguished from those forms by the presence of the longitudinal line and 

 central nodule. Although there are some free forms in this family, the 

 greater part of them, included in the genus ffomphonema, have their 



frustules either affixed at their bases 

 or attached to a stipes. This stipes 

 seems to be formed by an exudation 

 from the frustule, which is secreted 

 only during the process of self-divi- 

 sion : hence when this process has been 

 completed, the extension of the single 

 filament below the frustule ceases; 

 but when it recommences, a sort of 

 joint or articulation is formed, from 

 which a new filament begins to sprout 

 for each of the half-frustules; and 

 when these separate, they carry apart 



Gomphonema gnatum, more hhly the PeduncW which Support them, 



magnified: A, side view of frustule; B, front as far as their divergence can take 



- 



view; c, frustule in the act of self-division. 



th j g 



dichotomous character is given to the entire stipes (Fig. 186). The spe- 

 cies of Gomphonema are, with scarcely an exception, inhabitants of fresh 

 water, and are among the commonest forms of Diatomaceae. 



295. Lastly, we come to the large Family Naviculec& 9 the members of 

 which are distinguished by the symmetry of their frustules as well in the 

 lateral as in the front view, and by the presence of a median longitudinal 

 line and central nodule in both valves. In the genus Navicula and its 

 allies, the frustules are free or simply adherent to each other; whilst in 

 another large section they are included within a gelatinous envelope, or 

 are inclosed in a definite tubular or gelatinous frond. Of the genus Na- 

 vicula an immense number of species have been described, the grounds 

 of separation being often extremely trivial. Those which have a lateral 

 sigmoid curvature (Fig. 165) have been separated by Prof. W. Smith 

 under the designation Pleurosigma, which is now generally adopted; but 

 his separation of another set of species under the name Pinnularia 

 (which had been previously applied by Ehrenberg to designate the stri- 

 ated species), on the ground that its striae (costae) are not resolvable into 

 dots, was not considered valid by Mr. Ealfs, on the ground that in many 

 of the more minute species it is impossible to distinguish with cer- 

 tainty between striae and costa3. Mr. Slack has since given an account 

 of the resolution of the so-called costae of twelve species of Pinnularia 

 into beaded structures. 1 The beautiful genus 8tauroneis, which belongs 

 to the same group, differs from all the preceding forms in having the 

 central nodule of each valve dilated laterally into a band free from striae, 

 which forms a cross with the longitudinal band. 



296. The multitudinous species of the genus Navicula are for the most 

 part inhabitants of fresh water; and they constitute a large part of most 

 of the so-called ' infusorial earths' which were deposited at the bottoms of 

 lakes. Among the most remarkable of such deposits are the substances 



1 " Monthly Microscopical Journal," Vol. vi. (1871), p. 71. 



