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 (striæ), flutings, or networks, &c., arranged with great regularity and symmetry, often of extreme minuteness, and rendering them exquisite objects under the microscope. 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 appearance as regards colour.
To illustrate the forms and markings of the frustules and valves, we may select the following species taking first those which occur in fresh water.
In Epithémia tur´gida ([Pl. V.] fig. 30), the side-view or valve (s) exhibits transverse or slightly radiating lines, with intermediate rows of dots—these markings being continued over the margins of the valves so as to appear also in the front view (fig. 30 f), ceasing at the hoop. The frustules are curved or arcuate (ar´cus, a bow) in the side view, oblong and narrowed at the ends in the front view.
In Fragilária capucína ([Pl. V.] fig. 23), which is extremely common in fresh-water pools, &c., the frustules are united side by side into long filaments, which are often twisted. In the separate and prepared frustule, the front view ([Pl. V.] fig. 24 f) is rectangular, the valves (s) being narrowly lance-shaped or lanceolate. The valves under ordinary illumination appear colourless and without markings, but, by proper management of the light, very fine transverse striæ are seen upon them, consisting of rows of very minute dots. Fig. 24 s* represents the valve of Fragilaria vires´cens, a nearly allied species.
Diat´oma elongátum ([Pl. V.] fig. 31) is often found with the above. Its frustules are coherent at the angles. The front view (fig. 32 f) is rectangular, often slightly narrowed in the middle; and the valves are narrowly linear, and capitate at the ends; they are also transversely striated.
In Synédra splen´dens ([Pl. V.] fig. 5) the frustules radiate from a soft gelatinous cushion. They are linear in the front view (fig. 4 f), the valves (fig. 4 s) being gradually narrowed or attenuated from the middle to the ends, and exhibit transverse striæ interrupted opposite a middle longitudinal line.
In Campylodis´cus costátus ([Pl. V.] fig. 27) the frustules are disk-shaped and curved, so as somewhat to resemble a saddle. The markings consist of central dots, with radiating coarse flutings.
Nitzsch´ia minutis´sima ([Pl. V.] fig. 28) has oblique valves, i. e. the front half of the suture is not opposite the back portion; the valves (fig. 29) are constricted in the middle, and the ends narrowed and prolonged. The markings consist of a row of oblong dots or puncta (punctum, a point). This species often forms yellowish layers upon damp paths, &c.