WHITE SHRIMP FROM THE GULF OF MEXICO 



85 



trinsic and 1 intrinsic to the structure. The 

 wedge-shaped extrinsic abductor (figs. 46; 47, B) 

 originates on the ventromedial wall of the basipo- 

 dite. The muscle tapers to a point, laterally, in- 

 serting on the posterior margin of the exopodite 

 foramen. Contractions of the exopodite abduc- 

 tor turn the exopodite laterad. Intrinsic to the 

 flagellar exopodite is a long abductor muscle (fig. 

 46) which originates in the proximal region of 

 the exopodite and runs distally along the lateral 

 side of the flagellum. Its contractions increase 

 the lateral curvature of the flagellum, thus turn- 

 ing it laterad. The extrinsic exopodite abductor 

 muscle of Penaeus is represented under the same 

 name in Astacus and Callinectes. The exopodite 

 of Pandalus is extremely reduced. The intrinsic 

 exopodite abductor probably appears as the exop- 

 odite flagellum muscle in Astacus and Callinectes. 



EXPODITE ADDUCTOR MUSCLES OF 

 THIRD MAXILLIPED 



Figures 46; 47, B 



Two exopodite adductor muscles in Penaeus 

 turn the exopodite flagellum mesad. The conical 

 extrinsic adductor originates on the dorsomedial 

 wall of the basipodite, dorsad of the extrinsic 

 exopodite abductor muscle (fig. 46), and inserts 

 on the anterior rim of the exopodite foramen. 

 The intrinsic exopodite adductor is comprised of 

 a pair of small muscles originating on the pos- 

 terolateral side of the flagellum base and attach- 

 ing a short distance distally on the medial and 

 dorsal wall of the flagellum. The extrinsic ad- 

 ductor has a counterpart in Callinectes, but not 

 in Astacus. The little intrinsic adductor muscle 

 in Penaeus possibly may be the exopodite flagel- 

 lum abductor muscle of Astacus and Callinectes, 

 although this is doubtful. 



MEROPODITE FLEXOR MUSCLE OF THIRD 

 MAXILLIPED 



Figures 46, 47 



The meropodite flexor muscle is a long, spindle- 

 shaped muscle arising in the basipodite and pass- 

 ing distally along the lateral wall of the ischiopo- 

 dite to an apodemal insertion on the proximal end 

 of the meropodite. The apodeme of insertion is 

 located ventromedially. When the meropodite 

 flexor contracts, the meropodite and distal articles 

 of the endopodite are turned sharply mesad and, 



due to the arrangement of the condyles, ventrad. 

 The ischiopodite-meropodite junction is the major 

 functional joint in the third maxilliped. The 

 meropodite flexor muscle of Penaeus is fully ho- 

 mologous with the meropodite flexor muscle of 

 Astacus and Callinectes. The third maxilliped 

 endopodite of Pandalus has lost parts by fusion, 

 making difficult the homologies of its muscles. 



CARPOPODITB PRODUCTOB MUSCLE OF THIRD 

 MAXILLIPED 



Figure 46 



The carpopodite productor muscle takes origin 

 over an extensive area along the lateral side of the 

 meropodite (fig. 46) and inserts on an apodeme 

 of the carpopodite. Its contractions straighten the 

 carpopodite with respect to the meropodite. The 

 muscle is represented in the third maxilliped of 

 Astacus and apparently in Pandal-us. The produc- 

 tor muscle is referred to as an extensor by Cochran 

 in Callinectes due to a difference in condylic 

 orientation. 



CARPOPODITE REDUCTOR MUSCLE OF THIRD 

 MAXILLIPED 



Figure 46 



Opposing the action of the carpopodite produc- 

 tor muscle is the carpopodite reductor muscle. 

 This muscle originates in the proximal region of 

 the meropodite and passes distally along the me- 

 dial side of the meropodite to insert on a ventro- 

 medial apodeme of the carpopodite. Upon con- 

 traction, the muscle turns the carpopodite and the 

 distal articles ventrad. The carpopodite reductor 

 muscle of Penaeus is the same muscle in Astacus. 

 Pandalus. and Callinectes. although in the latter 

 Cochran describes it as a flexor. 



PROPODITE EXTENSOR MUSCLE OF THIRD 

 MAXILLIPED 



Figure 46 



Originating for some length along the lateral 

 margin of the carpopodite, the propodite extensor 

 muscle inserts on a laterally located apodeme of 

 the propodite. The muscle extends the propodite 

 and dactylopodite directly anterior to the carpo- 

 podite. The same muscle is found in all the crus- 

 taceans to which reference has been made. 



