THE CRAYFISH 119 



(the fourteenth and fifteenth of the whole series) are markedly 

 different in the two sexes. In the female the second ab- 

 dominal limbs resemble the third, but the first abdominal limbs 

 are very small, consisting of a small unjointed protopodite bear- 

 ing a single short imperfectly jointed filament, the representa- 

 tive of the endopodite. 



In the male both the first and second abdominal limbs are of 

 relatively large size, and are turned forwards so as to underlie the 

 bases of the last thoracic limbs. The second limb consists of a 

 protopodite of the normal structure bearing an exopodite and an 

 endopodite. The exopodite presents no remarkable characters, 

 but the endopodite is much modified, its proximal joint being 

 enlarged and its outer distal margin expanded into a thin plate 

 rolled up in the form of a scroll lying alongside the short distal 

 filament (fig. 26, XV). The first pair of abdominal limbs of the 

 male is still more modified. The coxopodite is present, but the 

 exopodite is absent, and the basipodite appears to be fused with 

 the endopodite. The proximal joint only of the latter is repre- 

 sented, and forms an expanded plate whose sides are rolled in 

 so as to form a canal open at both ends (fig. 26, XI V\ These 

 modified limbs serve as sperm conduits by means of which the 

 spermatozoa of the male are conveyed to their appropriate 

 position between the bases of the posterior thoracic limbs 

 of the female. 



The abdominal limbs of the crayfish do not present, at first 

 sight, any obvious resemblance to the foliaceous limbs of Apus, 

 nor, except for the imperfectly jointed filaments of the exo- 

 podites and endopodites, do they resemble the first thoracic 

 appendages of that animal to any great extent. But it should 

 be remarked that they -are comparable with the second 

 antennae and mandibles of the nauplius, having like them a 

 jointed basal piece or stalk bearing an outer and an inner 

 branch. . 



. Passing now to the thoracic limbs. The four pairs of walking 

 legs are long, slender, and made up of a series of seven joints. 

 The great chelae are stout, and the penultimate joint is expanded 

 and modified to form the pincers, but they too are made up 

 of a series of seven joints, and neither they nor the walking 

 legs show any trace of the division of the distal part of the 

 limb into an inner and an outer branch. 



But this feature reappears in the third pair of maxillipeds 



