20 THE NATUEAL 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. 3) ; 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. 3, 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 

 (fig. 3, 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 crayfish, 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 



