SOME BIOLOGICAL PROBLEMS. 13
that it suggests that the unusual exposure to light may have
something to do with the development of the pigment. In
Simocephalus vetulus there is usually a longitudinal row of three
brown or brownish-purple spots a little in front of the middle
of the valves, the centre spot being surrounded by a ring of a
more transparent greenish tint. There are also two spots nearer
the ventral margin of the valves more or less in line with the
first and second spots of the main row respectively. (In passing,
it may be mentioned that if the animal is viewed from behind
when lying on its back, as it often does, the ring with its centra
spot on each valve gives the body the superficial appearance
of the head of some insect with two large compound eyes.) The
best case, however, of characteristic colour pattern among the
Cladocera is seen in Latona setifera. In this species, in addition
to a certain amount of diffuse colour and minute flecks of red
and blue, there are veritable chromatophores of brown, red and
blue or violet, arranged in a very definite manner over the dorsal
and lateral parts of the valves. I believe that it is in such
simple cases as those just referred to that there is a chance of
determining the reason why the colour is deposited in the par-
ticular cells in which we find it, and so of solving the physio-
logical problem of the arrangement of colour in characteristic
patterns.
Some of the Ostracods also show very well-marked colour
schemes on their shells. For example in Cypria ophthalmica
the pattern consists of more or less regularly spaced brown
spots, in Cypridopsis vidua of dark transverse bands and in
Notodromas monacha of dark bands over the ventral flattened
area and obliquely across the ventral part of the valves. It
seems very probable that, as in the case of Scapholeberis mucro-
nata already alluded to, the coloration in the last-named species
is connected with its habit of attaching itself to the underside
of the surface-film by its ventral margin. The Copepods, al-
though as a rule very slightly and uniformly coloured, also
furnish a number of cases of bright coloration and characteristic
arrangements of the colour. Species of Diaptomus may be
brilliantly red or blue or partly black and partly white. Some
species of Cyclops are quite characteristically marked in various
colours, but the most intense coloration among Copepods is
shown by certain marine forms and what is specially noteworthy