WATER-SURFACE PLANTS AND ANIMALS. 231
relation of the mollusca to the surface-film requires further
investigation.
Some Planarians, especially Polycelis nigra, may often be
seen moving, or practically stationary, immediately in contact
with the surface-film, and, as they are heavier than water, it
would appear that they make use of the force of surface-tension
in some way for support. It is by no means evident, however,
how this is brought about and further observations and experi-
ments are required to elucidate this matter.
All the species of Hydra are to be seen at times hanging
down from the surface-film. In the case of these animals there
is no doubt that surface-tension is brought into play by means
of a water-repellent mucus produced by the breaking down of
the cells of the basal disc. In corroboration of the possibility
of this method of suspension, it may be mentioned that such a
substance as vaseline can be experimentally used to bring about
exactly similar results with small bodies slightly heavier than
water. Precisely how the animals leave the water plants to
which they normally adhere and move on to the surface-film,
has not been observed so far as I am aware.
I have little doubt that some Rotifers and Protozoa also
make use of the force of surface-tension for support, for I have
at various times seen a number of forms belonging to both
groups apparently suspended from or moving in contact with
the surface-film. The appliances hitherto available, however,
have not allowed of any precise observations being made as to
the exact way in which such support was obtained.
Next in importance to the utilisation of the surface-
film for support, is its exploitation by certain water-surface
plants and animals for keeping themselves partly or
wholly dry and incidentally in some cases for keeping
themselves the right way up. In the water-surface plants,
both the methods already evolved by many land plants for
the purpose of keeping their upper surfaces dry are to be found,
namely by means of a smooth waxy surface (Hydrocharis, Lamia,
Wolffia), or by a covering of water-repellent scales, ridges, hairs, &c.
(Azolla, Salvinia, &c.). In the former case whenever the upper
surfaces of the plants have been wetted by rain or splashing or
after actual submergence, the water on the smooth water-repel-
lent surface has, owing to surface-tension, a strong tendency to