8 THE NATURAL HISTORY OF THE COMMON CRAYFISH. 
females, the others the males, And the latter may 
be 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. 3, A; 
15, 16). In the female, there are mere soft filaments in 
the place of the first pair of styles (fig. 3, B; 16). 
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 
easterly 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 
burrows which 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 
