14
THE ESSEX NATURALIST.
is that in some of these cases the colour is structural, i.e., due
to the structure of the plumose setae on the appendages and
not to pigment of any kind.
Among the many ecological problems that might be illus-
trated by reference to the Entomostraca I will only refer to one,
namely, the effects upon the animals of the direct action of
particular elements in their environment. Whether such action
is of any importance in the production of new species is, as you
all know, a matter of considerable discussion even now. But
there can be no question about its importance to the existing
-animals themselves and one advantage of a consideration of
this matter is that it does at least suggest lines of experimental
work. The best known work on these lines is probably that
of Harrison and Garrett on the inheritance of melanism in cer-
tain moths induced by the addition of lead or manganese to
the diet of the caterpillars. As regards the Entomostraca, Pro-
fessor Agar, experimenting with Simocephalus vetulus, found
that by feeding these animals upon a particular culture of proto-
phyta, the ventral parts of the valves became reflexed so that,
as seen from the front, all the appendages were fully exposed.
This in itself is a very interesting phenomenon, but the further
remarkable fact was ascertained that the young of individuals
which had been removed to control conditions before the eggs
were laid, showed the same abnormality as their parents had
acquired as a direct result of their environment. The ab-
normality was not even confined to the first brood, but occurred
in later broods and even in the next generation though in a
rapidly diminishing degree. Using the same species, but sub-
jecting it to a much higher temperature than usual, Professor
Agar found that this change in conditions was followed by a
marked reduction in the size of the animals and that this
character also was exhibited by the young of individuals trans-
ferred to normal conditions before the eggs were laid. Such
cases as these can probably be matched by what is occasionally
found in nature. For instance, it is strongly suspected that the
higher temperature is the cause of the elongation of the head
of the plankton Daphnias in the summer and also of the reduction
in size of these and some other forms such as Bosmina longi-
rostris. These cases of the modifying action of some element in
the environment cannot, however, be regarded as genuine