130 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The second difference follows from the first. We 

 find muscles in the tail fan of Penaeus, similar to 

 those in Pandalus and Astacus, that have (mite 

 dissimilar functions. These have been renamed in 

 accordance with their presumed functions in 

 Penaeus. The tail fan of the white shrimp ap- 

 pears to be operated by- 27 muscles, including cer- 

 tain rectal muscles not found in Astacus or Panda- 

 lus. This compares with 18 muscles in Astacus 

 and 13 in Pandalus. 



UROPOD RBMOTOR MUSCLES 



Figure 73 



Three uropod remotor muscles are found in the 

 white shrimp. One originates dorsomedially in 

 the posterodorsal corner of the sixth abdominal 

 segment. The second arises slightly anterior to 

 the first and runs posteroventrad to join the first 

 on a common apodeme. The third originates 

 lateroventrad of the first two and joins the com- 

 mon remotor apodeme at a point caudad of the 

 first two remotor muscles. The common remotor 

 apodeme now runs a short distance ventrocaudally 

 to insert in heavy connective tissue beneath the 

 dorsal rim of the uropod protopodite. Contrac- 

 tions of the muscles bring the uropods dorsad, in 

 opposition to the action of the large abdominal 

 muscles attached to the uropods. The two dorsal 

 uropod remotor muscles are perhaps homologous 

 to the medial uropod protopodite remotor muscle 

 of Pandalus and Astacus. while the ventrolateral 

 remotor is considered to be the homolog of the 

 medial protopodite remotor muscle in the coon 

 stripe shrimp and the crawfish. 



PROTOPODITE REMOTOR MUSCLE OF UROPOD 



Figures 70, 73 



The protopodite remotor muscle is a short, 

 thick structure originating in the same tendinous 

 connective tissue upon which the uropod remotors 

 insert. The protopodite remotor thus functional- 

 ly prolongs the remotor muscle series to the cau- 

 dal edge of the protopodite. The only muscle 

 that looks similar to the protopodite remotor mus- 

 cle of Penaeus is the exopodite reductor muscle of 

 Pandalus, a muscle not found in Penaeus. 



DORSAL UROPOD ROTATOR MUSCLES 



Figures 70, 72, 73, 74 



Due to the presence of transverse fasciae divid- 

 ing them, three dorsal uropod rotator muscles are 



found in each half of the sixth abdominal segment 

 of Penaeus (fig. 74, A). These large muscles 

 originate over much of the dorsomedial and lat- 

 eral areas of the sixth abdominal tergum to the 

 margin of the external arm of the anterior oblique 

 muscle (fig. 73, B). The dorsal uropod rotators 

 pass ventrocaudally to insert upon three branches 

 of a large apodemal tendon shared with the telson 

 flexor muscles. The tendon arises from the ven- 

 tral rim of the protopodite at its junction with the 

 sixth abdominal segment. On contraction, the 

 uropod rotators turn the lateral side of the proto- 

 podite downward, thereby maintaining the uro- 

 pods in the most advantageous position for draw- 

 ing the animal backwards through the water. 

 The dorsal uropod rotator muscles of Penaeus ap- 

 pear to be homologous with those of Astacus and 

 Pandalus. 



VENTRAL UROPOD ROTATOR MUSCLE 



Figures 71, 72 



The small ventral uropod rotator muscle origi- 

 nates by a broad apodeme attached to the postero- 

 ventral sternum of the sixth abdominal segment 

 (fig. 71). The muscle runs caudally and laterally 

 to insert by a strong tendon into the medial edge 

 of the protopodite. Contractions of the muscle 

 rotate the protopodite about its long axis, thus 

 bringing the lateral edge of the protopodite and 

 exopodite downwards. The ventral uropod rota- 

 tor muscle of Penaeus is not shown in the studies 

 of Pandalus. A ventral uropod rotator is in- 

 dicated by Schmidt in Astacus. The muscle in the 

 crawfish is much larger than its counterpart in 

 Penaeus. 



PROTOPODITE ROTATOR MUSCLE 



Figure 74 



The protopodite rotator muscle is revealed by 

 the removal of the thick exopodite abductor muscle 

 located in the lateral part of the protopodite (fig. 

 73, B). The muscle takes origin in the same con- 

 nective tissue providing insertions for the uropod 

 remotors and the origin of the protopodite remotor 

 muscle. The protopodite rotator passes laterally 

 and ventrally to an area of insertion on the ventro- 

 lateral surface of the protopodite. The counter- 

 part of the muscle in Astacus and Pandalus is not 

 clear. 



