By weighting the tow-net it may be used at various depths to capture the floating animals of mid-water. When it is so used, however, it is impossible to tell from what depth any particular specimen may have come, since it may have been captured during the hauling in of the net. For more precise investigations in deep water, "closing tow-nets" of various types have been devised, which can be opened by a "messenger" sent down the line when the net has reached the desired depth, and closed again by another "messenger" before it is hauled in. A simple method that has proved very successful for collecting small Crustacea living on a sandy bottom in shallow water is to employ a light tow-net with a cane ring, and with a heavy sinker attached to the towing line at a distance of a few feet in front of the net. As the sinker is dragged along the bottom, the net floats up behind it, and catches small animals stirred up by its passage. For collecting the smaller fresh-water Crustacea--Water-fleas and the like--a small muslin ring-net may be used in ponds and ditches. The plankton of the open water of lakes is best obtained by means of a tow-net like that described above for use in the sea. The interesting blind species known as "Well Shrimps" are to be looked for in the water of springs and wells. In wells fitted with a pump, Professor Chilton found that "the Crustacea are often brought up most abundantly when pumping is first commenced, and that jerking the handle of the pump somewhat violently is often more successful than pumping at the ordinary rate." In disused open wells, they may be trapped by baiting a muslin ring-net with a piece of stale meat or fish, and pulling it up rapidly after it has remained in the well for a few hours. The subterranean waters of caves have yielded many curious species in various parts of the world. For the capture of species living in the deep water of large lakes, a special form of dredge has been devised with runners to prevent it from sinking into the soft mud, while the mouth of the net is raised a few inches above the bottom. For preserving Crustacea the best medium to use is 70 per cent. alcohol. Strong spirit diluted with a little less than one-third its bulk of water gives about the required strength. If too strong spirit is used, the specimens tend to be hard and brittle, and delicate organisms become shrivelled. Methylated spirit as sold in the shops in this country contains mineral naphtha, and turns milky when water is added, so that it is unsuitable for preserving specimens. Methylated alcohol without naphtha can be bought, by permission of the Inland Revenue authorities, but only in considerable quantities at a time.