12
THE ESSEX NATURALIST.
these simpler organisms should offer the most favourable material
for the investigation of the problem of the connection between
the colour pattern and the bodily structure and function. It is
from this point of view that I think a consideration of some of
the cases of the occurrence of colour and pattern among the
Entomostraca should be useful, although, so far as I know,
not much use has, up to the present, been made of these animals
for the investigation of this matter.
As a general rule the Cladocera are not very decidedly
coloured, most of the species being of a nearly uniform tint,
ranging in different cases from an almost hyaline transparency
(e.g., Leptodora kindtii) through varying shades of yellow to
brown. Fairly frequently, however, other colours, more par-
ticularly red and red-violet, are present, either diffused throughout
all the tissues or more or less localised. Some species are always
noticeably red (e.g., Ceriodaphnia laticaudata and Ilyocryptus
sordidus), while others are only occasionally so (e.g., Daphnia
pulex). It is also a very common thing for animals belonging
to this Order either to develop some colour or to have their
usual colours intensified during the sexual period, i.e., usually,
in the autumn. A very good example of this is Polyphemus
Pediculus which, when in what Weismann called its "Hochzeits-
kleid" (wedding garment) is most gorgeously attired in nearly
all the colours of the rainbow. It was because of this rich display
of colour in this and some other species of Cladocera during
the period when males and females producing eggs requiring
fertilisation were in evidence, that Weismann contended that
we had here a case of the intensification of colour by the aid of
sexual selection, but this idea is not now generally accepted.
In only a few cases can members of this Order be said to show
colour patterns in their shells. The simplest case of all is pro-
bably that of Chydorus globosus in which there is a large dark
brown area in the middle of each valve. In Scapholeberis mucro-
nata the main coloration, which is very dark brown, almost
black, is sharply confined to the ventral third of the valves and
to a small patch on the ventral face of the head. This arrange-
ment is no doubt related to the animal's habit of moving about
attached to the surface-film by its ventral margin, but this fact
does not help very much in solving the physiological problem
why the colour should be found where it is, except, perhaps,