The sudden change in appearance presented to the eye as the frustule is seen to roll over is rather peculiar. As a rule, therefore, we must examine all specimens in every aspect, to accomplish which very shallow cells should be selected, say of 1⁄100th of an inch deep, and covered with glass 1⁄250th of an inch thick. A good penetrating objective should be used, and careful illumination obtained. The Diatomaceæ are perhaps more widely distributed than any other class of infusorial life; they are found in fresh, salt, and brackish water; many grow attached to other bodies by a stalk ([Plate II]., No. 33, Licmophora and Achnanthidium); while others, as Pleurosigma, No. 40, swim about freely.
PLATE XI.
DIATOMACEÆ, RECENT AND FOSSIL.
There are a considerable number of Diatomaceæ which, when in the young state, are enclosed in a muco-gelatinous sheath; while others are attached by stipes or stalk to algæ. It would be vain, in a limited space, to attempt a description of this numerous and extensive family. Nägeli and other observers describe a “mucilaginous pellicle on the inner layer of the valves,” while, as Menghine observes, “an organic membrane ought to exist both inside and outside, for the silica could not become solid except by crystallizing or depositing itself on some pre-existing substance.” The surface of the frustules is generally very beautifully sculptured, and the markings assume the appearance of dots (puncta), stripes (striæ), ribs (costæ), pinnules (pinnæ), of furrows and fine lines; longitudinal, transverse, and radiating bands; canals or canaliculi; and of cells or areolæ; whilst all present striking varieties and modifications in their form, character, and degree of development. Again, the fine lines or striæ of many frustules are resolvable into rows of minute dots or perforations, as occur in Pleurosigma angulatum, delineated in the accompanying microphotograph ([Fig. 294]), taken for the author purposely to show the markings on this especially selected test diatom.
Fig. 293.
1. Pleurosigma attenuatum; 2. Pleurosigma angulatum; 3. Pleurosigma Spencerii. Magnified 450 diameters.
The nature of the markings on the diatom valves is one of considerable interest, and attempts have been made to produce them artificially, but without success.
