8
THE NATURAL HISTORY OF THE COMMON CRAYFISH.
females, the others the males.
And the
latter may
he still more easily known by the possession of four curved styles, attached to the under face of the first two rings of the tail, which are turned forwards between the hinder legs, on the under side of the body (fig. 8, A; 15, 16).
In the female, there are mere soft filaments in
the place of the first pair of styles (fig. 8, B ; 15). Crayfishes do not inhabit every British river, and even where they are known to abound, it is not easy to find them at all times of the year.
In granite districts and
others, in which the soil yields little or no calcareous matter to the waters which flow over it, crayfishes do not occur.
They are intolerant of great heat and of
much sunshine ; they are therefore most active towards the evening, while they shelter themselves under the shade of stones and banks during the day.
It has been
observed that they frequent those parts of a river which run north and south, less than those which have an easterty and westerly direction, inasmuch as the latter yield more shade from the mid-day sun. During the depth of winter, crayfishes are rarely to be seen about in a stream; but they may be found in abundance in its banks, in natural crevices and in burrowswhich they dig for themselves.
The burrows
may be from a few inches to more than a yard deep, and it has been noticed that, if the waters are liable to freeze, the
burrows are deeper and further from
the surface than otherwise.
Where the soil, through
