WATER-SURFACE PLANTS AND ANIMALS. 229
for support, we have two excellent examples in the genera
Scapholeberis and Notodromas. The former belongs to the
Cladocera and the latter to the Ostracoda. In each case very
definite modifications are to be found enabling the animals to
make use of the surface-film. In the species of Scapholeberis
the ventral part of the valves, instead of being convex, as in most
of the Cladocera (Daphnia, Simocephalus, etc.), is straight
and the setae on its flattened margins are modified, at least
in 5. mucronata, into a series of peculiar overlapping scales.
It is this ventral margin which is applied to the surface,
some of the scale-like seta breaking through the surface-
film and, owing no doubt to their water-repellent nature,
giving rise to minute capillary depressions. Although very
small, the upward pull of these depressions is evidently
sufficient to counteract the slight difference in weight between
the animals and water. They thus remain suspended as long
as they wish, moving slowly about by means of occasional
strokes of their swimming antennae and feeding all the time
on any minute organic particles on the surface. When they wish
to descend they can at once break contact with the surface-
film by a more powerful and probably differently directed stroke
of the same antenna;. In Notodromas the ventral margin of
the shell is also straight and flattened with special chitinous
ridges. Here again it is this flattened ventral margin which is
applied to the surface-film when the animal wishes to remain
at the surface, the water-repellent ridges, or parts of them,
piercing the surface-film and producing the necessary capillary
depressions. Although belonging to such different groups of
the Crustacea as the Cladocera and the Ostracoda, the modifi-
cations connected with the use of the surface-film are singularly
alike in these two genera. It is also noteworthy that both
have a considerable amount of dark pigment on the ventral
parts of the valves as if to render them less conspicuous when
seen from above by, say, the Whirligig beetles, Pond-skaters, etc.
The other kinds of Entomostraca utilising the force of
surface-tension for support seem to do so only occasionally
and, so to speak, incidentally, showing no evident modifications
for the purpose. The very common Cladoceran Simocephalus
vetulus can support itself by breaking through the surface-
film with the same two setae on its swimming antennae with which