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 very 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. 87, 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, the 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 
