34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



and function. It originates on the outer walls of the carpopodite, nar- 

 rowing to an insertion on the outer proximal edge of the propodite. 



108. Musculus extensor propoditis III pedis vmxillaris (fig. ii). — 

 Originating on the inner proximal walls of the carpopodite and inserted 

 by a tendon on the inner proximal corner of the propodite, this muscle 

 is like the corresponding one in the preceding segment in form and 

 function. 



lop. Musculus fJe.vor dactylopoditis III pedis maxillaris (fig. 1 1) . — 

 This muscle originates on the outer proximal border of the propodite 

 and is inserted by a tendon on the outer proximal edge of the dac- 

 tylopodite. Relative to the size of its opposing extensor, it is better 

 developed than any other flexor in this endopodite, and apparently can 

 exert a strong outward pull upon the dactylopodite. 



no. Muscidus extensor dactylopoditis III pedis maxillaris (fig. 

 ii).- — Originating on the inner proximal margin of the propodite, this 

 muscle is inserted on the inner proximal edge of the dactylopodite, 

 which is brought strongly downward by it. In this segment the ex- 

 tensor and the flexor are nearly the same in size and apparent strength. 



THE PEREIOPODS 



The five pairs of pereiopods, or true legs, occur upon the last five of 

 the eight thoracic segments. The promotor, the remotor, and the 

 levator muscles of each pereiopod are extrinsic in the origin of all 

 their parts. The depressor, of the telopodite, however, is both extrinsic 

 and intrinsic in origin, for the larger and heavier branches originate 

 in the body wall or some of its apodemes, while there are usually two 

 or more branches originating proximally on the anterior and posterior 

 walls of the coxopodite. 



The functions of the diflferent pairs of legs become evident upon 

 examining their distal segments. On the first pair of legs, the dactylo- 

 podite arises on the anterior (preaxial) border of the propodite nearly 

 at the middle ; the unhampered tip of the propodite curves and tapers 

 to a point, while the dactylopodite curves in a way to oppose it effec- 

 tively, the two forming a powerful pinching claw, the chela, which is 

 rendered still more effective by the horny teeth that have developed 

 on the opposable surfaces. The claw is held out in front of the cara- 

 pace, and may swing widely forward and sidewise in a horizontal 

 plane, and less widely in a perpendicular plane, both movements serv- 

 ing as the means to reixilse an enemy or to seize and tear up food. The 

 extension of the leg forward has caused it to assume a position half- 

 turned from the normal one, and now the true anterior (preaxial) sur- 

 face of the first pereiopod is uppermost. 



