Common Crayjish. 209 



u. Second joint of posterior thoracic appendag-e, or ' maxilliped/ 

 or ' foot-jaw/ This joint, as in the large cheliferous append- 

 age V 1, next behind it, represents two joints, the basipodite 

 and the isehiopodite of the normal seven-jointed endopodite, 

 as seen in v 2, v 3, v 4, v 5. Its antero-internal edge is den- 

 ticulate^ as in the Lobster, but its serratures are concealed 

 by a fringe of setae. 



V I. First abdominal appendage, modified terminally by the pro- 



duction of the distal outer angle of its penultimate joint or 

 propodite so as to form a pair of pincers with the opposed 

 last joint, or dactylopodite. Two other joints, the ' carpo- 

 podite' and ' meropodite/ are shown in this figure ; the two 

 basal joints are not seen. The two first abdominal legs are 

 not symmetrically developed in the Crayfish, nor in the 

 Marine Lobster, foreshadowing thus the extreme inequality 

 seen in the Hermit-Crabs. 



V 2 and V 3. Second and third pairs of abdominal or ambulatory 



legs, differing from v i in their smaller size, and in not having 

 the second and third joints fused. It is at the interval 

 between the first and second joints, the 'coxopodite' and the 

 ' basipodite,' that the power of casting off a limb, in conse- 

 quence of a fright or injury, is put in play by Crustacea. 



V 4 and V 5. Fourth and fifth pairs of the ambulatory legs of the 



Decapod. The two terminal joints do not form pincers, 

 otherwise they resemble v 2 and v 3. The vas deferens opens 

 in the coxopodite or basal joint of ?? 5. 



w I and w 2. The appendages of the two first post-abdominal seg- 

 ments modified so as to form an accessory copulatory organ. 



w 3, w 4, and w 5. Appendages of the third, fourth, and fifth post- 

 abdominal segments, consisting each of two basal joints, 

 which serve as a pedicle to two multiarticulate filaments 

 representing an cxopodite and an endopodite. The inner of 

 these two filaments has its first joint longer and larger than 

 the other joints in either filament. This greater relative 

 importance of the endopodite is more plainly seen in the 

 antennae and thoracic limbs, where the exopodite is markedly 

 smaller than the endopodite, and most plainly in the abdo- 

 minal limbs where it is absent, or only represented by the 

 constriction marking the third joint, or ' isehiopodite.' 

 p 



