Some slight difference in treatment is required by certain species, as that of _Asplanchna priodonta_; after the application of the cocaine solution, which should be added slowly, that is, by letting a few drops trickle down the side of the live-trough; this, being heavier than water, sinks to the bottom, thus narcotising the rotifers, and assisting to kill them with the cilia fully expanded. They should be left quietly for fifteen minutes, then thoroughly washed with distilled water. On further experimenting, Mr. Rousselet found that a weaker solution of osmic acid alone, 1/4 per cent., answers quite as well as, if not better than, Flemming’s fluid; even this must be allowed to act for only a very short time--a minute at most; the rotifers then remain white and transparent, excepting the ova, in which a fat-like substance, _lecithene_, is secreted. If they become too much stained, they may be decolourised by passing them through peroxide of hydrogen. For narcotising the following solution has been found most useful:--Take a 2 per cent. solution of cocaine hydrochlorate, 3 parts; methylated spirit of wood naphtha, 1 part; and distilled water, 6 parts. This must be added as before directed, drop by drop, watching the effect upon the rotifers under the microscope. All the rotatoria may be killed and preserved in the same way. For mounting, Mr. Rousselet prefers a slightly _hollowed-out_ glass cell, the advantage of which is that the rotifers are kept to the centre, and cannot move to the edge. A little difficulty at first presents itself to exclude air-bubbles, but this, with a little care, can be overcome by placing a drop of a two or three per cent. solution of formalin, just sufficient to fill the cell. Then transfer the rotifers with a dipping pipette to the cell, and lower the cover-glass down very gently, removing any excess of fluid by blotting-paper. The best cement for the cover-glass is gold-size. _Method of Cementing._--After many years’ experience, I have arrived at the conclusion that for cementing down the cover-glass there is nothing better than either gold size or gum dammar varnish. The latter, for some preparations, will be improved by the addition of a small proportion of indiarubber dissolved in naphtha. (See Appendix.) Should glycerine be preferred, carefully wash away any surplus quantity by gently syringing; then apply a ring of waterproof cement round the cover-glass. An inexpensive one can be made by dissolving ten grains of gum-ammoniac in an ounce of acetic acid, and adding to this solution two drachms of Cox’s gelatine. This liquid flows easily from the brush and is waterproof, rendered more so if subsequently brushed over with a solution of ten grains of bichromate of potash in an ounce of water. An especial recommendation to this cement is its adhesiveness to glass, even should there be a little glycerine left behind on the cover. After the gelatine ring is thoroughly dry any kind of cement may be employed.