20 THE NATURAL HISTORY OF THE COMMON CRAYFISH. 
the region of the head, in front, from that of the thorax 
behind. 
The thorax seems at first not to be jointed at all; but 
if its under, or what is better called its sternal, surface is 
examined carefully, it will be found to be divided into as 
many transverse bands, or segments, as there are pairs of 
legs (fig. 8); and, moreover, the hindermost of these 
segments is not firmly united with the rest, but can be 
moved backwards and forwards through a small space 
(fig. 8, B; xiv). 
Attached to the sternal side of every ring of the abdomen 
of the female there is a pair of limbs, called swimmerets. 
In the five anterior rings, these are small and slender 
(tig. 8, B; 15, 19); but those of the sixth ring are very 
large, and each ends in two broad plates (20). These 
two plates on each side, with the telson in the middle, 
constitute the flapper of the erayfish, by the aid of which 
it executes its retrograde swimming movements. ‘The 
small swimmerets move together with a regular swing, 
like paddles, and probably aid in propelling the animal 
forwards. In the breeding female (B), the eggs are 
attached to them; -while, in the male, the two anterior 
pairs (A; 15, 16) are converted into the peculiar styles 
which distinguish that sex. 
The four pairs of legs which are employed for walking 
purposes, are divided into a number of joints, and the 
foremost two pairs are terminated by double claws, 
arranged so as to form a pincer, whence they are said to 
