114 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



substantially modified from the typical plan of 

 the preceding anterior oblique muscles. 



At the posterior end of the last segment, the 

 sixth anterior oblique muscle has three areas of 

 insertion. Moving caudad from the anterior end 

 of the sixth segment, certain fibers of the sixth 

 anterior oblique muscle insert in tough connective 

 tissue over a large area on the ventral surface of 

 the sixth somite (figs. 71, 72). These fibers are 

 obviously associated with the flexion of the sixth 

 abdominal segment on the fifth. The major por- 

 tion of the ventral fibers of the sixth anterior 

 oblique continue caudad beyond the sternal inser- 

 tion area to insert on a strong tendon, or apodeme 

 of anterior oblique muscle (figs. 71, 72). This 

 apodeme is firmly connected to the uropod pro- 

 topodite, the base element from which the uropods 

 arise. Thus the sixth anterior oblique flexes the 

 uropods as well as the sixth segment. 



The third area of sixth anterior oblique inser- 

 tion is slightly dorsad of the ventral anterior ob- 

 lique apodeme. It, too, is apodemal, and, while 

 lighter than the ventral apodeme, is a strong ten- 

 don. This dorsal anterior oblique apodeme is best 

 seen in median sagittal aspect (figs. 61 ; 74, B) . In 

 contrast to the ventral anterior oblique apodeme, 

 the dorsal tendon bifurcates anteriorly into two 

 parts. Two large and distinct portions of the 

 sixth anterior oblique muscle insert on the tendi- 

 nous bifurcations. The two parts of the dorsal 

 apodeme fuse and run caudad to a point on the 

 ventrolateral surface of the telson. The sixth an- 

 terior oblique muscle clearly flexes the telson in 

 addition to its previously mentioned activities. 



The sixth anterior oblique muscle of Penaeus is 

 undoubtedly homologous with that of Pandalus 

 and Astacus. Berkeley and Schmidt indicate the 

 presence in the latter forms of the ventral apodeme 

 of the anterior oblique muscle. They do not men- 

 tion the dorsal apodeme of this muscle. 



POSTERIOR LOOP OF ANTERIOR OBLIQUE MUSCLE 

 OF ABDOMEN 



FlGUKES 60, 61 



Arising from the posterior portion of the dorso- 

 medial area of the anterior oblique muscle, where 

 the bilateral pairs of the latter are fused at the 

 midline, is the posterior loop of the anterior ob- 

 lique muscle. The muscle runs ventrocaudally, 

 laterad of the central muscle, and then passes 

 mesad to join the transverse muscle. At the mid- 



line, the fibers of the posterior loop connect with 

 those of its partner on the other side of the seg- 

 ment. Each anterior oblique muscle of Penaeus 

 appears to have a posterior loop. 



EXTERNAL ARM OF ANTERIOR OBLIQUE 

 MUSCLE OF ABDOMEN 



Figures 60, 63, 64 



The external arm of anterior oblique muscle is 

 much larger and more important than the pos- 

 terior loop. The external arm connects with the 

 broad dorsal end of the anterior oblique muscle 

 at the midline (fig. 61). The muscle sweeps over 

 the central muscle dorsally and then runs antero- 

 ventrad along the outside of the abdominal 

 muscles to a ventrolateral point just inside the 

 next anterior abdominal somite (fig. 60). Daniel 

 (1931c) considers this point the origin of the ex- 

 ternal arm, and he notes that the area is the com- 

 mon insertion of the anterior and posterior 

 obliques of an anterior segment. To summarize, 

 each external arm originates on the posteroven- 

 tral edge of an abdominal segment and traverses 

 very nearly the whole of a segment to its insertion 

 on the main body of the anterior oblique muscle. 

 Taking the anterior oblique muscle as a whole, 

 including its external arm, the muscle function- 

 ally traverses three segments from the ventral 

 surface of one, over the central muscle of the sec- 

 ond, dorsally. and hence to the ventral surface of 

 the third segment. 



As one would expect, the abdomen of Penaeus 

 contains 6 external arms accompanying 6 anterior 

 oblique muscles (fig. 60). Its counterparts in the 

 abdomen of Pandalus and Astacus are not clear. 



POSTERIOR OBLIQUE MUSCLES OF ABDOMEN 



Figures 61, 64 



The abdomen of Penaeus has five posterior 

 oblique muscles, beginning with the first abdom- 

 inal segment. The posterior obliques are asso- 

 ciated with the anterior obliques and function with 

 the latter in the flexion of the abdomen. Except 

 for the first posterior oblique, the muscles arise 

 dorsal to the transverse muscles in close association 

 with the head or anterior end of the central mus- 

 cles. The posterior oblique then runs ventrocau- 

 dally through two segments to insert ventrad in 

 common with its accompanying anterior oblique 

 muscle ( fig. 61 ) . The first posterior oblique origi- 

 nates in tendinous tissue dorsal to the anterior 



