276 



THE MICROSCOPE AND ITS REVELATIONS. 



surface. In some Diatoms, as Surirella (Fig. 175) and Campylodiscus 

 (Fig. 176), these interruptions are connected with what were thought, 

 by Prof. W. Smith, to be minute canals hollowed out between the sili- 

 cified casing and the membrane investing the endochrome; but the ap- 

 parent canals are really internal ribs, or projections of the shell, showing 

 its characteristic i beaded' structure under sufficiently good objectives. 

 In many genera the surface of each valve is distinguished by the pres- 

 ence of a longitudinal band on which the usual markings are deficient, 

 and this is widened into small expansions at the extremities, and some- 

 times at the centre also, as we see in Pleurosigma (Fig. 164) and Gom- 

 plionema (Fig. 186); but this band is merely a portion in which the sili- 

 cified casing is thicker than it is elsewhere, forming a sort of rib that 

 gives firmness to the valve, its expansions being solid nodules of the same 

 substance. These nodules were mistaken by Prof. Ehrenberg for aper- 

 tures; and in this error he has been followed by Klitzing. There cannot 

 any longer, however, be a doubt as to their real nature. 



276. The nature of the delicate and regular markings with which 

 probably every Diatomaceous valve is beset, has been of late years a sub- 

 ject of much discussion among Microscopists; but on certain points 

 there is now a general convergence of opinion. There can be no longer 

 any question as to the nature of the comparatively coarse areolation seen in 

 the larger forms, such as Istlimia (Fig. 163), Triceratium (Fig. 164), 

 and BiddulpMa (Fig. 167); in all of which that structure can be dis- 

 tinctly seen under a low magnifying power and with ordinary light. In 



each of these instances, we see a 

 number of symmetrically disposed 

 areolce, rounded, oval, or hexago- 

 nal, with intervening boundaries; 

 and these have now been unmis- 

 takably proved to be depressions, 

 lying in the interspaces of an 

 elevated reticulation. The retic- 

 ulation presents itself in clear 

 relief, when viewed Binocularly 

 with a sufficiently high power; and 

 the depression of its interspaces 

 becomes manifest when an edge- 

 view ig obtained of a curved BUT- 

 highly m^I- facej snch ag tbafc of a valye of 



Isthmia. 1 Both the depressed 

 areolse and the intervening network of Diatoms presenting this areo- 

 lation, when examined with a sufficient magnifying power, show 

 the < beaded ' aspect characteristically displayed in Pleurosigma angula- 

 tum (Fig. 166); and this is also well seen in some species of Aotinocyclus 

 and Coscinodiscus, and in the beautiful Heliopelta (Plate I, Fig. 3). 

 The observations of Mr. Stephenson on Coscinodiscus oculus Iridis ( 



1 When specimens of Diatoms which exhibit this areolation are examined 

 by the test of Focal adjustment ( 152), it is found that if they are mounted in 

 Canada balsam, the optical effects are reversed; the areolaB being made to look 

 bright (like elevations) when the distance of the objective is increased, and dark 

 when it is diminished This, however, is readily explicable by the fact that the 

 refractive power of the Balsam is greater than that of the Silicified valve; so 

 that the predominant effect will be produced by the convexities formed in the 

 medium by the concavities of the object. (See Schultze in "Quart. Journ. of 

 Microsc. Science," Vol. iii., N.S., 1863, p. 131.) 



