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THE ESSEX NATURALIST.
it hooks itself on to water plants, etc. The Copepod Cyclops
fuscus (and probably other species of the same genus) can also
be found hanging from the surface-film and this it does
apparently by breaking through with two of the long curved setae
on its second antennae.
The Mollusca to be seen at the surface more or less regularly
include most if not all species of the air-breathing forms, notably
those belonging to the genera Limnaea, Planorbis and Physa.
With regard to these animals there are two complications which
make it rather difficult to decide to what extent they actually
make use of the surface-film for support. In the first place they
are often somewhat lighter than water owing to their possession
of air-sacs. They can, however, upon occasion, certainly be
heavier than water, even when at the surface, because of the
control which they have over the size of their air-sacs. They
are then in the same position as the non-air-breathing Molluscs,
such as Bithynia, Hydrobia and even Sphaerium and Pisidium,
which naturally are always heavier than water and yet may
be found occasionally at the surface. But in all cases it is
the foot which is applied to the surface and this introduces
the second complication because this organ can be depressed
in the centre to form a shallow boat which would be sufficient
to explain the flotation without recourse to surface-tension.
Nevertheless, there are times when the foot in contact with the
surface-film remains flat, except for the slight wave-like move-
ments which pass over its surface, and when the air-breathing
species are also heavier than water and yet remain suspended.
That they are making use of the surface-film for support in these
cases is practically certain, but it is still somewhat of a mystery
how it is done. The cilia with which the foot is clothed may
have something to do with it, but on the whole I am inclined
to think that it is the mucus, which is secreted so abundantly
by all molluscs, that is responsible for the phenomenon ; not,
however, as has been suggested, by acting as a buoyant float,
for the mucus is certainly heavier than water, but by its being
water-repellent and so capable of producing capillary depres-
sions in the surface-film. My own experiments have certainly
shown that the mucus can, under some conditions, be water-
repellent and that it can be brought up from below the surface
and attached to the surface-film. But the whole subject of the