WHITE SHRIMP FROM THE GULF OF MEXICO 



131 



UROPOD STATOR MUSCLES 

 Figures 73. 74 



The uropod stator muscles are 2 or 3 small mus- 

 cles whose function appears to be the retention of 

 the exopodite and endopodite in position with re- 

 spect to one another. The affinities of these 

 muscles in other crustaceans are not known. 



EXOPODITE ABDUCTOR MUSCLES 

 Figures 70. 71, 73 



The uropod of Penaeus contains 4 exopodite ab- 

 ductor muscles, all varying widely in shape and 

 size. The anteriormost is a short, strong muscle 

 occupying the lateral part of the protopodite (fig. 

 73, B) . The muscle originates in the anterior cur- 

 vature of the protopodite and inserts on the lat- 

 eral margin of the exopodite. The muscle turns 

 the exopodite laterally about its dorsoventral con- 

 dyles. Also intrinsic to the protopodite are two 

 smaller exopodite abductor muscles inserting in 

 common with the first abductor. The small abduc- 

 tors lie beneath the protopodite remotor muscle 

 and originate at two points on the ventromedial 

 surface of the protopodite. The fourth and caudal 

 exopodite abductor muscle, best seen in dorsal view 

 (fig. 70), runs along the lateral margin of the exo- 

 podite. The muscle originates distally and inserts 

 on a lateral apodeme of the protopodite. The 

 function of the exopodite abductors is to spread 

 the tail fan. 



The first and fourth exopodite abductor muscles 

 of Penaeu& are homologous with the dorsal and 

 lateral exopodite abductors of Antaeus and with 

 the lateral abductor muscle of Partdalus. The 

 relationships of the two small abductors in Pen- 

 aeus are not clear. The small ventral uropod 

 exopodite abductor muscle shown by Schmidt in 

 Astacus does not appear in Penaeus. 



EXOPODITE ADDUCTOR MUSCLES 



Figures 70, 71, 73, 74 



The uropod exopodite of Penaeus is operated 

 by three exopodite adductor muscles. The first 

 exopodite adductor is rostral to the other two. 

 It is a short, strong, twisted muscle originating 

 on the posterodorsal surface of the protopodite 

 (fig. 70). The first exopodite adductor passes 

 ventrocaudally through other muscles to an area 

 of insertion on the anteroventral surface of the 



exopodite (fig. 71). The second exopodite ad- 

 ductor muscle is the longest uropod muscle. The 

 structure lies in the midline of the exopodite, 

 inserting on the posterodorsal margin of the pro- 

 topodite beneath the first adductor. The muscle 

 takes origin in the distal region of the exopodite 

 and on a little fascia dividing the main body of 

 the muscle from a small, lenticular muscle (fig. 

 7(1). This little distal muscle is thought to be the 

 vestige of a muscle operating the distal exopodite 

 joint. 



The third exopodite adductor lies along the 

 medial margin of the exopodite. The muscle is 

 made up of short fibers attaching to a long apo- 

 deme running through the middle of the structure. 

 The apodeme arises from the posterodorsal mar- 

 gin of the protopodite. Contractions of the exo- 

 podite adductors turn the exopodite towards the 

 midline of the animal, opposing the abductors. 



The first exopodite adductor muscle of Penaeus 

 appears in Pandalus as the anterior exopodite ad- 

 ductor. The second exopodite adductor found in 

 the white shrimp is not represented in either 

 Pandalus or Astacus. The third adductor of 

 Penaeus is fully homologous with the muscle 

 known as the posterior exopodite adductor in 

 Pandalus and the uropod exopodite adductor 

 muscle in the European crawfish. 



EXOPODITE PRODUCTOR MUSCLE 



Figures 71, 72, 74 



The exopodite productor muscle of Penaeus is 

 a broad, flat structure occupying the ventral part 

 of the protopodite (figs. 71, 72). The productor 

 originates in the heavy connective tissue of the 

 large apodeme common to the dorsal uropod ro- 

 tator muscles and the telson flexor muscles. The 

 productor muscle passes caudad to a broad in- 

 sertion on the ventral rim of the exopodite. The 

 exopodite productor muscle is represented in the 

 tail fan of Pandalus, but its relationships in the 

 crawfish are not known. No reductor of the 

 exopodite has been found in the tail fan of the 

 white shrimp. 



ENDOPODITE ADDUCTOR MUSCLE 



Figures 70, 71, 73. 74 



Originating in the distomedial part of the endo- 

 podite, the endopodite adductor muscle runs 

 proximally in the endopodite to its point of in- 



