Diatoms are the most common of the eukaryotic algae. They belong among the heterokonts along with the golden and brown algae, as shown by the pigmentation of the chloroplasts and the structure of the gametes, although these have lost the mastigonemes found in other such groups. Most diatoms are single cells and live between two silicate shells, one overlapping the other like the two halves of a petri dish. When the creature divides, each half keeps one side and grows a new shell within it, meaning that smaller and smaller diatoms are produced. Once a certain minimal size is reached, the cell undergoes meiosis to produce heterokont gametes, which fuse to form a zygote that divides and releases new large diatoms.
The diatoms are divided into two main groups: the pennate diatoms, which are bilaterally symmetrical, and the centrate diatoms, which are radially symmetric. The latter appear to be paraphyletic to the former. Both groups show a wide diversity of forms, some quite beautiful and ornate. A few diatoms form radial or filamentous colonies. Most diatoms are non-motile but some are capable of an oozing motion. Fossil diatoms are found as far back as the early Cretaceous, and some rocks (called diatomaceous earth) are composed almost entirely of them.
Living diatoms are often found clinging in great numbers to filamentous algae, or forming gelatinous masses on various submerged plants. Cladophora is frequently covered with Cocconeis, an elliptically shaped diatom; Vaucheria is often covered with small forms. Other algae will pay for examination, especially if they look brown. If stones in the water have a brown, slippery coating, you can be sure of diatoms. Sometimes the brown coating on sticks and stones is so abundant that it streams out with the current. If rushes and stems of water plants have a brown, gelatinous coating, you are likely to find millions of specimens of the same diatom. The surface mud of a pond, ditch, or lagoon will always yield some diatoms. They can be made to come out from the mud by putting a black paper around the jar and letting direct sunlight fall upon the surface of the water. The diatoms, in a day or even less, will come to the top in a scum which can be easily secured.
Since diatoms form an important part of the food of molluscs, tunicates, and fishes, the alimentary tracts of these animals often yield deep-water forms which are not easily secured in any other way. Fresh-water diatoms appear in greatest abundance in spring, are comparatively scarce in summer, and reappear in autumn, though not so abundantly as in the spring. Marine forms can be secured by scraping barnacles, oyster shells, and other shells.
The silicious shells of diatoms are among the most beautiful objects which could be examined with the microscope. To obtain perfectly clean mounts requires considerable time and patience, but when the material is once cleaned, preparations may be made at any time with very little trouble.
Note : Much of this text (from Living diatoms on) is from Methods in Plant Histology from the 1900s. Handle with care!