LAND AND FRESH WATER MOLLUSCA. 173
H. sericea, great care is needed, so as not to remove the delicate
hairs or bristles which clothe the surface of the epidermis (or perios-
tracum), a layer of animal tissue with which the shells are covered.
Care must be taken also to preserve the opercula of such genera as
Paludina and Cyclostoma. The operculum should be detached from
the "foot" of the snail, the interior of the shell plugged with cotton
wool, and the operculum gummed down in its natural position.
The shells of mussels and other bivalves which gape a good deal
after the animal has been removed, should be carefully closed and
bound with thread until dry. Bivalves as small as Sphaerium corneum
may be treated in this way, but the smaller species of Pisidium
and some of the smaller univalves, as, for instance, the little sedge-
shell Carychium minimum, may be dried in hot sand. Care, how-
ever, is required in the process, since too much heat will cause a
transfusion of the carbonaceous matter of the animal into the sub-
stance of the shell, and so discolour it.
Slugs require a different treatment. On this subject, Mr. Tate,
in his "British Mollusks" says, "as regards the internal shell
it may be obtained by making a conical incision in the shield,
taking care not to cut down upon the calcareous plate, which
can then be removed without difficulty. The animals can only be
conserved by keeping them in some preservative fluid ; but the great
object to keep in view is to have the slug naturally extended. Most
fluids contract the slugs when they are immersed in them. A writer
in the Naturalist gives a process for the preservation of slugs,
which he states to answer admirably, and to be very superior to spirit,
glycerine, creosote, and other solutions:—' Make a cold saturated
solution of corrosive sublimate; put it into a deep wide mouthed
bottle, then take the slug you wish to preserve and let it crawl on a
long slip of card. When the tentacles are fully extended, plunge it
suddenly into the solution; in a few minutes it will die, with the
tentacles fully extended in the most life-like manner, so much so,
indeed, that if taken out of the fluid it would be difficult to say
whether it be alive or dead. The slugs thus prepared should not be
mounted in spirit, as it is apt to contract and discolour them. A
mixture of one and a half parts of water and one part of glycerine
has been found to be the best mounting fluid ; it preserves the colour
beautifully, and its antiseptic qualities are unexceptionable. A good-
sized test tube answers better than a bottle for putting them up, as it
admits of closer examination of the animal. The only drawback to