WHITE SHRIMP FROM THE GULF OF MEXICO 



113 



transverse muscles crossing the midline. Their 

 great size at the midline is partly due to the ad- 

 dition of fibers of the posterior loop of the anterior 

 oblique muscle, as well as those of the ventral slip 

 of the central muscle, mentioned above. The 

 muscles function in the fulcral support of the cen- 

 tral muscles, together with lateral compression of 

 the abdomen. The transverse abdominal muscles 

 of Penaeus are fully homologous with those of 

 Pandalus and Astacus. Both of the latter forms 

 have six of these muscles. 



ANTERIOR OBLIQUE MUSCLES OF ABDOMEN 



Figures 60 to 64 



The anterior oblique muscles are directly re- 

 sponsible for the strong abdominal flexions of 

 which the white shrimp is capable, and as such 

 are on functional grounds the most important 

 longitudinal muscles of the abdomen. Each an- 

 terior oblique muscle is made up of several parts. 

 The thickest part runs mesad of the central mus- 

 cle and is closely applied to its opposite number 

 at the median sagittal line (fig. 61). From the 

 midline, the main part of the anterior oblique 

 turns ventrally and curves caudad to an insertion 

 area two segments to the rear. The area of its 

 insertion is on the posteroventral margin of the 

 abdominal segment concerned. Penaeus has 6 

 anterior oblique muscles, compared with 7 each 

 for Pandalus and Astacus. 



FIRST ANTERIOR OBLIQUE MUSCLE OF ABDOMEN 



The first anterior oblique muscle arises in con- 

 nective tissue in the fifth or last thoracic segment, 

 mesad of the lateral thoracoabdominal muscle 

 (fig. 61) and runs ventrocaudally through part 

 of the last thoracic and all of the abdominal seg- 

 ment to an area of insertion on the posterior 

 edge of the first abdominal segment (fig. 64). 

 The muscle functions to pull the ventral surface 

 of the first abdominal segment forwards about its 

 hinges with the fifth thoracic segment. The 

 muscle appears in the abdomen of Pandalus and 

 Astacus. 



SECOND ANTERIOR OBLIQUE MUSCLE OF 

 ABDOMEN 



This muscle (fig. 61) traverses the first and 

 second abdominal segments, inserting on the pos- 

 teroventral portion of the second segment (fig. 



64). By the contractions of the second anterior 

 oblique muscle the ventral part of the second ab- 

 dominal segment is brought rostrad. Homologs 

 of this muscle have been described in Astacus and 

 Pandalus. 



THIRD ANTERIOR OBLIQUE MUSCLE OF ABDOMEN 



Beginning in the second abdominal segment 

 (fig. 61), the third anterior oblique muscle sweeps 

 posteroventrad and inserts on the posteroventral 

 rim of the third abdominal segment. It pulls the 

 ventral part of the third segment forward. The 

 same muscle is found in the abdomen of Pandalus 

 and Astacus. 



FOURTH ANTERIOR OBLIQUE MUSCLE OF 

 ABDOMEN 



From its origin in the third abdominal seg- 

 ment, the fourth anterior oblique runs directly 

 ventrad, then turns sharply caudad and passes 

 along the ventral surface of the abdomen to its 

 insertion on the fourth segment (fig. 61). The 

 muscle in Penaeus is represented in Astacus and 

 Pandalus. 



FIFTH ANTERIOR OBLIQUE MUSCLE OF ABDOMEN 



This muscle runs in a gentle curve from its dor- 

 sal origin in the fourth abdominal segment to its 

 insertion in the rear of the fifth segment (fig. 61). 

 The medial manifestation of the fifth anterior 

 oblique is much slenderer than is that of the pre- 

 ceding anterior oblique muscles. Contractions of 

 the muscle flex the fifth segment ventrad with re- 

 spect to the fourth segment. Schmidt and Berke- 

 ley describe the muscle in Astacus and Pandalus. 



SIXTH ANTERIOR OBLIQUE MUSCLE OF ABDOMEN 



Like the fifth anterior oblique, the sixth curves 

 gently from the dorsal part of the fifth abdominal 

 segment to its special insertions at the caudal end 

 of the long sixth segment (fig. 61). The ventral 

 part of the sixth anterior oblique muscle lying in 

 the sixth segment may be seen to be very thick. 

 Actually, its cross-sectional area is no greater 

 than the caudal portions of the other anterior 

 oblique muscles, but by virtue of its concentration 

 along the midline, a median sagittal view reveals 

 a large portion of the muscle. The muscle is 

 functionally similar to the preceding anterior 

 oblique muscles. However, its insertions upon the 

 sixth segment and the parts of the tail fan are 



