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 ; 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 

 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 

 burrowswhich they dig for themselves. The burrows 

 may be from a few inches to more than a }^ard 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 



