DIATOM ACE ;E. 77 



When the frustules are examined in the living 

 state, the cell- contents resemble those of ordinary 

 vegetable cells, excepting in regard to the colour, and 

 exhibit granules and globules, and sometimes a nucleus 

 is visible. It will also be noticed that many of the 

 free frustules move slowly across the field of the 

 microscope ; but the cause of the motion is unknown. 



When the frustules have been properly prepared, 

 the surface of the valves exhibits a number of coarser 

 or finer markings, consisting of dots, lines (striae), 

 flutings, or networks, &c., arranged with great re- 

 gularity and symmetry, often of extreme minuteness, 

 and rendering them exquisite objects under the mi- 

 croscope. The exhibition of these markings requires 

 not only that the valves shall be properly prepared 

 and mounted, but that the object-glasses be of good 

 quality, and that the management of the light be 

 thoroughly understood ; so that to a beginner, their 

 examination is often a matter of great difficulty ; for 

 only the very coarsest or largest of these markings 

 can be perceived in the natural frustules. 



The appearance of these markings, and even their 

 apparent absence or presence, frequently depends 

 upon the kind of illumination used : thus, under one 

 kind of illumination the valves may appear simply 

 white or coloured, while under another they appear 

 covered with lines, and under a third with dots. It 

 will often be observed, also, that the colour of the 

 valves varies according to the illumination and the 

 power used the same valve appearing white, yellow, 

 brown, blue, &c; and the wet or dry state of the 

 frustules often cause a decided difference in their ap- 

 pearance as regards colour. 



To illustrate the forms and markings of the frus- 

 tules and valves, we may select the following species 

 taking first those which occur in fresh water. 



In Epithemia tur'gida (PI. V. fig. 30), the side- 

 view or valve (s) exhibits transverse or slightly radia- 



H 3 



