12. _Desmidiaceæ_.--Microscopic gelatinous plants, of a screen color, growing in fresh water, composed of cells devoid of a silicious coat, of peculiar forms such as oval, crescentic, shortly cylindrical, cylindrical, oblong, etc., with variously formed rays or lobes, giving a more or less stellate form, presenting a bilateral symmetry, the junction of the halves being marked by a division of the green contents; the individual cells being free, or arranged in linear series, collected into fagot-like bundles or in elegant star like groups which are embedded in a common gelatinous coat. Reproduced by division and by resting spores produced in sporangia formed after the conjugation of two cells and union of their contents, and by zoospores formed in the vegetative cells or in the germinating resting spores. 13. _Diatomaceæ_.--Microscopic cellular bodies, growing in fresh, brackish, and sea water: free or attached, single, or embedded in gelatinous tubes, the individual cells (frustules) with yellowish or brown contents, and provided with a silicious coat composed of two usually symmetrical valves variously marked, with a connecting band or hoop at the suture. Multiplied by division and by the formation of new larger individuals out of the contents of individual conjugated cells; perhaps also by spores and zoospores. 14. _Volvocineæ_.--Microscopic cellular fresh water plants, composed of groups of bodies resembling zoospores connected into a definite form by their enveloping membranes. The families are formed either of assemblages of coated zoospores united in a definite form by the cohesion of their membranes, or assemblages of naked zoospores inclosed in a common investing membrane. The individual zoospore-like bodies, with two cilia throughout life, perforating the membranous coats, and by their conjoined action causing a free co-operative movement of the whole group. Reproduction by division, or by single cells being converted into new families; and by resting spores formed from some of the cells after impregnation by spermatozoids formed from the contents of other cells of the same family. [Illustration: MALARIA PLANTS COLLECTED AT 165TH STREET, EAST OF 10TH AVENUE, OCT., 1881. Plate IX.--Large group of malaria plants, Gemiasma verdans, collected at 165th Street, east of 10th Avenue, New York, in October, 1881, by Dr. Ephraim Cutter, and projected by him with a solar microscope. Dr. Cuzner--the artist--outlined the group on the screen and made the finished drawing from the sketch. He well preserved the grouping and relative sizes. The pond hole whence they came was drained in the spring of 1882, and in August was covered with coarse grass and weeds. No plants were found there in satisfactory quantity, but those figured on Plate VIII. were found half a mile beyond. This shows how draining removes the malaria plants.]