88 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



condyles. To the coxopodite is attached the 

 strong, curving tube of the basipodite. The axis 

 of the condyles between the coxopodite and ba- 

 sipodite is rotated 90° from the axis of the con- 

 dyles connecting the coxopodite and the ventral 

 skeleton. Primarily by means of the basipodite, 

 the distal elements of the pereiopod are raised and 

 lowered. A sharp process projects distally from 

 the distomedial portion of the basipodite. A 

 small, fingerlike exopodite bearing long setae 

 projects laterad from the basipodite. 



The ischiopodite is hinged to the basipodite to 

 permit limited reduction of this article, and also 

 some rotation of the distal elements due to the 

 oblique angle of the condyles. Like the basipo- 

 dite, the ischiopodite bears a sharp spine medially. 

 The meropodite, one of the longer articles of the 

 first pereiopod, articulates with the distal end of 

 the ischiopodite, allowing some flexion and ex- 

 tension, together with a little reduction, of the 

 distal segments. The most movable joint in the 

 first walking leg is that connecting the meropodite 

 and the carpopodite. The axis of the joint is hori- 

 zontal, and the carpopodite is thereby capable of 

 deep flexion on the meropodite. An antennal 

 cleaning brush composed of comb-like setae re- 

 sides in a distomedial depression of the carpopo- 

 dite. 



The chela is freely movable on the carpopodite. 

 The propodite component is made up of a base 

 element and a distal process (fig. 49). The dac- 

 tylopodite articulates with the distal part of the 

 base of the propodite, opposing the distal propo- 

 dite process. Extremely fine calcareous teeth on 

 the inner margins of the propodite process and 

 dactylopodite enhance the cutting ability of the 

 chela. Chemosensory pits from which project 

 chemoreceptor tufts of fine setae are distributed 

 in rows about the propodite process and dactylo- 

 podite surface. 



MUSCLE ELEMENTS 



The musculature of the first pereiopod is 

 closely similar to that of the third maxilliped and 

 the remaining walking legs. It is also similar to 

 the pereiopod musculature of other decapods. 

 The first pereiopod of Penaeus contains at least 

 21 discrete muscles arranged in 14 functional 

 muscle types. Astacus has 19 muscles of 13 types, 

 and Callinectes has 20 muscles functioning in 11 

 ways. The third pereiopod of Pandalus contains 

 16 muscles of 13 types. 



COXOPODITE PROMOTOR MUSCLES OF FIRST 

 PEREIOPOD 



Figs. 49, 50, 51 



The first pereiopod of Penaeus possesses at least 

 two coxopodite promotor muscles. They origi- 

 nate by broad regions along the anterodorsal 

 margin of the pleural plate belonging to the seg- 

 ment and insert on the anterior rim of the cox- 

 opodite. Their contractions turn the coxopodite 

 forward. The coxopodite promotors of Penaeus 

 are represented in Astacus, Pandalus, and Calli- 

 nectes. 



COXOPODITE REMOTOR MUSCLE OF FIRST 

 PEREIOPOD 



Figs. 49, 50, 51 



The strong, spatulate coxopodite remotor mus- 

 cle originates on the posterodorsal part of the 

 laterotergal plate and attaches to a small apodeme 

 on the posterior margin of the coxopodite fora- 

 men. The muscle turns the coxopodite rearward 

 and in so doing brings the distal elements of the 

 pereiopod ventrad, in opposition to the coxopo- 

 dite promotors. The movement is evidently 

 weaker than that of promotion, since the single 

 remotor is much smaller than the promotor 

 muscles. The homologies of the coxopodite re- 

 motor muscle of Penaeus with the remotor muscle 

 of Callinectes are fairly obvious. Astacus, how- 

 ever, has three remotors. Which of these is homol- 

 ogous with the coxopodite remotor of Penaeus 

 is not certain. 



BASIPODITE LEVATOR MUSCLES OF FIRST 

 PEREIOPOD 



Figures 49, 50, 51 



A characteristic of pereiopod musculature is 

 the great weight and strength of the basipodite 

 musculature, affording powerful levation and de- 

 pression of that article and with it the distal 

 elements of the leg. The situation in the Decapoda 

 is reminiscent of the trochanteral musculature 

 of the insects, except that the levators and de- 

 pressors of the latter originate within the coxa, 

 rather than on the pleural wall. Functionally, 

 the basipodite depressors are the major support 

 of the body between the walking legs. 



The first walking leg of Penaeus has 4 basipod- 

 ite levator muscles, of which 3 originate within 

 the coxopodite and 1 dorsal] y on the pleural wall. 



