Desmids (Desmideae) are a group of conjugate Algae (see ALGÆ) related to diatoms, but readily distinguished from these by their bright green instead of brownish-yellow colour, their cellulose instead of silicified walls, and their general form, which is usually more or less deeply constricted in the middle line. As with diatoms the lower forms are, however, united in long chains, so indicating a relationship to the lower and filamentous Conjugatae (Spirogyra, &c.). About four hundred species have been described, but these are often variable and ill defined. All are inhabitants of fresh water, but chiefly occur in the standing pools of heaths and peat-mosses. They rarely form large aggrega- tions; but usually occur associated with filamentous algæ, diatoms, and other forms of microscopic life. On account of the singular beauty of their general form they are much prized by the microscopist; their mode of division, too, is remarkable, yet akin to that of diatoms, the two halves of the cell being pressed apart at the median constriction by the development of two new bud-halves to complete them (see fig.), two new desmids being thus gradually formed, which become as symmetrical as the original one, yet with the two apparently similar halves of very unequal age. Conjugation also occurs, with formation of a resting spore; the cyst of which may have peculiar markings or hook-like prominences. See Ralfs' Desmidea; Micrographic Dictionary, &c.