94 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



apodeme attached to the lateral margin of the 

 dactylopodite. The muscle opens the jaws of the 

 chela. The dactylopodite abductor muscle of 

 Penaeus is fully homologous with the muscle of 

 the same name in the tirst pereiopod of Astacus 

 and Callinectes. The tirst pereiopod chela of Pan- 

 dalus is reduced. 



5b. Fifth Pereiopod 



The fifth pereiopod has been chosen as an ex- 

 ample of a nonchelate limb. It is truly ambula- 

 tory, slender and very long, much longer than the 

 first pereiopod, mainly due to pronounced length- 

 ening of the meropodite and carpopodite. Dorsad 

 of the fifth pereiopod projects a single pleuro- 

 branchia. 



SKELETAL ELEMENTS 



The coxopodite (fig. 54) is a rounded box whose 

 lateral corner projects dorsally to a point. This 

 point makes contact with the dorsal condyle of 

 the article. The ventral condyle is located on the 

 sternal plate between the limbs. The arrange- 

 ment of these coxopodite dicondyles permits this 

 article great freedom of movement. A large 

 region of thin cuticle, the articular cuticula (fig. 

 54), lying caudad of the coxopodite, presents 

 little resistance to posterior movements of the seg- 

 ment. A second articular cuticula associated with 

 basipodite movements is found on the lateral sur- 

 face of the coxopodite. 



The strong basipodite (fig. 54) is hinged to the 

 ventral surface of the coxopodite in the typical 

 way, allowing for extensive movements of this 

 article and of the distal segments in the vertical 

 plane. With the dorsolateral surface of the basip- 

 odite is articulated a small exopodite. The 

 ischiopodite (fig. 52) makes an oblique connec- 

 tion with the basipodite. The joint is slightly 

 movable. The ischiopodite may be bent ventrad 

 on horizontally located condyles. The articula- 

 tion point between the short ischiopodite and the 

 long meropodite is a transverse one capable of 

 limited motion. By means of the joint the mero- 

 podite may be turned ventrad. 



The carpopodite articulates with the meropo- 

 dite by means of a complex ''knee" joint made up 

 of a pair of heavy condyles so arranged that the 

 joint has considerable freedom of action. Simi- 

 larly oriented, but not so freely movable, is the 

 articulation between the carpopodite and propo- 



dite. The propodite is not so long as the carpopo- 

 dite. The slender, tapering dactylopodite makes 

 contact with the propodite by a simple dicondylic 

 joint. Chemoreceptor tufts and other sensory and 

 mechanical setae are arranged in rows on the 

 dactylopodite and to some extent on the propodite. 



MUSCLE ELEMENTS 



The arrangement of the muscles of the fifth 

 pereiopod is typical of the true supporting limbs 

 of the thorax. The ambulatory muscles are light 

 by comparison with those of the heavy reptant 

 crustaceans and the terrestrial arthropods. In re- 

 sponse to the support function of this appendage, 

 the depressor musculature of the basipodite is es- 

 pecially well developed. The only anatomical ac- 

 count of the fifth pereiopod of a crustacean known 

 to the present writer is that of Cochran on Cal- 

 linectes. However, the fifth leg of the blue crab is 

 the swimming leg and hence its muscles, particu- 

 larly the basal ones, have diverged from the typi- 

 cal pattern. A remarkable uniformity of endopo- 

 dite musculature between Penaeus and Callinectes 

 still remains, however. The fifth pereiopod of 

 Penaeus is operated by 24 muscles grouped into 13 

 functional types. 



COXOPODITE PROMOTOR MUSCLES OF FIFTH 

 PEREIOPOD 



Figures 52, 53, 55 to 57 



Two coxopodite promotor muscles are found in 

 the fifth pereiopod of Penaeus. The smaller lat- 

 eral promotor originates by its broad, dorsal, fan- 

 shaped portion on the laterotergal plate of the seg- 

 ment (figs. 55, 56) and inserts on the anterior rim 

 of the coxopodite. Lying medial to the latter mus- 

 cle is a larger promotor muscle mass originating 

 somewhat dorsad of the smaller promotor on the 

 pleural region. Contractions of these muscles 

 turn the coxopodite forward on its condyles. 



COXOPODITE REMOTOR MUSCLES 

 OF FIFTH PEREIOPOD 



Figures 52, 53, 55, 56 



The fifth pereiopod of Penaeus has two coxo- 

 podite remoter muscles. These are large, flat 

 muscles (fig. 55) situated beneath the thin ma- 

 terial of the articular cuticula (fig. 54). Both 

 originate by broad margins on the pleural plate 

 and insert on the caudal margin of the coxopodite. 



