124 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



action should be avoided. Moreover, the applica- 

 tion of the term "reductor" to the musculus reduc- 

 tor basipoditis in Pandalus is questionable, since 

 the muscle does not function as a flexor of the 

 basipodite. 



The basipodite remotor muscles apparently are 

 not represented in the pleopod of Astacus, Cam- 

 ba/us, or Callinectes, although a trace of these im- 

 portant muscles in Penaeus may have remained 

 during the evolution of the former animals as the 

 basipodite reductor muscle (musculus reductor 

 basipoditis). 



EXPODITE ROTATOR MUSCLES 



Figures 65, 66, 67, 69 



The pleopod basipodite of Penaeus setiferus 

 contains two muscles which rotate the exopodite. 

 The elongate origin of the fan-shaped lateral ex- 

 opodite rotator muscle is on the lateral surface of 

 the basipodite and may be identified easily 

 through the cuticle in preserved material (fig. 65). 

 The muscle becomes narrow as it runs distomesi- 

 ally to insert on the mesial surface of the complex 

 articular element joining the basipodite to the 

 exopodite. The muscle functions to square the 

 plane of the oar blade of the exopodite at the end 

 of the recovery stroke for the catch of the follow- 

 ing propulsive stroke. The mesial exopodite ro- 

 tator muscle (fig. 69) is a small muscle located in 

 the anterodistal lobe of the basipodite. Its con- 

 tractions apparently aid in feathering the exo- 

 podite oar blade. These muscles do not appear 

 in any of the crustaceans to which reference has 

 been made, although the mesial exopodite rotator 

 muscle may exist in Pandalus as the musculus ad- 

 ductor endopoditis. 



EXOPODITE EXTENSOR MUSCLES 



Figures 67 to 69 



The third pleopod of Penaeus setiferus is pro- 

 vided with 3 exopodite extensor muscles, 2 of 

 which originate in the basipodite and 1 in the exo- 

 podite. The largest of these has its origins along 

 the entire anterior margin of the basipodite (figs. 

 67, 68). It is one of the largest muscles in the 

 pleopod. The muscle inserts on an apodeme of 

 the exopodite articular element. Inserting on the 

 same apodeme is a slender exopodite extensor mus- 

 cle which is located just posterior to the anterior 

 exopodite extensor muscle described above (figs. 



68, 69). The posterior exopodite extensor has its 

 origin on the mesial surface of the basipodite (fig. 

 lis ) . When the two extensor muscles contract they 

 draw the articular element of the exopodite dor- 

 sad. The plane of articulation of this element is 

 such that the exopodite is swung down from its 

 lateral position in the propulsive stroke to a fully 

 extended position. At the same time the articular 

 element rotates the exopodite on its longitudinal 

 axis, with the aid of the mesial exopodite rotator 

 muscle, to feather the exopodite oar blade. 



That the two exopodite extensor muscles located 

 in the pleopod basipodite of Penaeus are missing 

 in Astaeus, Oambarus, and Callinectes is not sur- 

 prising in view of the extensive rearrangements 

 in the swimming appendages of the latter form. 

 The pleopod basipodite of Pandal/us has retained 

 at least the larger of the exopodite extensor mus- 

 cles of Penaeus, as the musculus adductor exopodi- 

 tis III pedis spurii. 



The third exopodite extensor (fig. 69) lies along 

 the mesial edge of the exopodite, attaching on the 

 margin of the exopodite articular element. Its 

 contractions serve to extend the exopodite and to 

 straighten the leading edge of the exopodite dur- 

 ing the recovery stroke. The muscle is homologous 

 with the exopodite flagellum muscle described by 

 Schmidt and Hart (1952) in Astacura, and by 

 Berkeley in Pandalus. It is missing in Calli- 

 nectes. 



EXOPODITE FLEXOR MUSCLES 



Figures 68, 69 



Two exopodite flexor muscles appear in the 

 third pleopod of Penaeus setiferus. One originates 

 proximally on the medial surface of the basipodite 

 (tig. 68), runs the length of the basipodite, and 

 inserts on the posterior surface of the exopodite 

 articular element (fig. 69). The muscle functions 

 to flex the exopodite laterally to place the exopo- 

 dite oar blade in position for the propulsive stroke. 

 The proximal exopodite extensor muscle of 

 Penaeus is in all likelihood the homolog of the 

 musculus abductor exopoditis of the third pleo- 

 pod of Pandalus. 



The distal exopodite flexor muscle is intrinsic 

 to the exopodite. The muscle is arranged along 

 the lateral margin of the exopodite (fig. 69) and 

 attaches to the posterior part of the exopodite ar- 

 ticular element. The contractions of the muscle 

 reinforce the flexing action of the proximal flexor 



