146 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



joints ; and of two terminal oval plates, which represent 

 the endopodite (en.p) and the exopodite {ex.p). The 

 latter is divided by a transverse suture into two pieces ; 

 and the edge of the larger or basal moiety is beset with 

 short spines, of which two, at the outer end of the series, 

 are larger than the rest. 



The second somite is longer than the first (fig. 1) ; it 

 has verjr broad pleura, while those of the first somite are 

 small and hidden by the overlapping front margins of the 

 pleura of the second somite. 



In the female, the appendages of the second somite of 

 the abdomen are similar to those of the third, fourth, and 

 fifth somites ; but in those of the first somite (fig. 37, B), 

 there is a considerable variation. Sometimes, in fact, 

 the appendages of this somite are altogether wanting ; 

 sometimes one is present, and not the other ; and 

 sometimes both are found. But, when they exist, these 

 appendages are always small; and the protopodite is 

 followed by only one imperfectly jointed filament, which 

 appears to represent the endopodite of the other ap- 

 pendages. 



In the male, tbe appendages of the first and second 

 somites of the abdomen are not only of relatively large 

 size, but they are widely different from the rest, those of 

 the first somite departing from the general type further 

 than those of the second. In the latter (C, C ') there is 

 a protopodite {cx.p, bp) with the ordinary structure, and 

 it is followed by an endopodite (en.p) and an exopodite 



