108 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



tern with the superficial ventral thoracic and the 

 superficial ventral thoracoabdominal muscles con- 

 sidered above in the section on the gnathothorax. 



Like their thoracic counterparts, the superficial 

 ventral abdominal muscles are very thin. Two of 

 them are found on each side of the ventral nerve 

 cord between the first and second and between 

 the second and third abdominal segments. Only 

 one is found on each side thereafter. Except for 

 the ventral muscle that passes between the fifth 

 and sixth abdominal segments, and which is ven- 

 trad of the nerve cord, all of them arise antero- 

 laterally and run mesad. In this respect, the 

 situation in Pandalus is much more like that of 

 Penaeus than is Astacus, The superficial ventral 

 muscles presumably function to hold the articular 

 cuticle between the abdominal segments in place. 



The superficial ventral muscles of Pandalus, 

 Astacus, and Callinectes are probably homologous 

 with the same muscles in Penaeus. Some differ- 

 ences exist. All of the superficial ventral muscles 

 of the former crustaceans are single on a side, 

 except for the last two abdominal segments of 

 Astacus in which the muscles are double. 



SUPERFICIAL LATERAL ABDOMINAL MUSCLES 



Although these muscles are not illustrated here, 

 their centers are indicated by small triangles on 

 figure 60. The triangles represent apodemal de- 

 pressions. Each superficial lateral abdominal 

 muscle is attached to broad, tough apodemal ma- 

 terial arising at the triangles. The muscles are 

 very thin, fan-shaped structures, and apparently 

 function to retain the position of the cuticle 

 during movements of the white shrimp. 



SUPERFICIAL DORSAL ABDOMINAL MUSCLES 



Figure 62 



Lateral to the midline on the dorsal surface of 

 the abdomen lie the thin superficial dorsal abdomi- 

 nal muscles. The muscles arise in superficial con- 

 nective tissue in the abdominal segments and pass 

 to apodemal material at the anterior margins of 

 the succeeding segments. Those inserting on the 

 fourth, fifth, and sixth abdominal segments are 

 square, flat muscles, situated laterad of the main 

 dorsal abdominal muscles. The superficial dorsal 

 abdominal muscle attached to the second abdomi- 

 nal somite is a long, thin muscle lying dorsad of 

 the main dorsal muscles. Inserting on the third 

 abdominal segment are two superficial dorsal ab- 



dominal muscles. The medial one is long and 

 slender, while the lateral muscle is rectangular 

 and has a lateral anterior projection. The super- 

 ficial dorsal muscles probably hold the articular 

 cuticle in position. 



The superficial dorsal abdominal muscles have 

 counterparts in Pandalus and Astacus. Those of 

 Pandalus, however, are all single muscles on each 

 side of the midline. In addition Berkeley illus- 

 trates a sixth superficial dorsal muscle in Panda- 

 Jus inserting on the telson. In Astacus, 6 pairs 

 of strong superficial dorsal muscles are found 

 lateral to the midline and a single seventh muscle 

 attached to the telson. 



MAIN DORSAL ABDOMINAL MUSCLES 

 Figures 60, 61, 62 



The main dorsal abdominal muscles function as 

 abdominal extensor muscles in opposition to the 

 action of the huge ventral abdominal muscle mass. 

 The dorsal abdominal muscles make apodemal 

 connection with the dorsal thoracoabdominal 

 muscles (fig. 61) and as such represent a func- 

 tional abdominal continuation of the latter. The 

 dorsal abdominal muscles may be divided into two 

 groups, the dorsolateral abdominal muscles, and 

 the dorsomedial abdominal muscles. Both groups 

 are easily distinguished from the underlying ven- 

 tral muscles. Their removal exposes the midgut 

 and various dorsal circulatory and nervous ele- 

 ments. The arrangement of the dorsolateral and 

 dorsomedial muscles of Astacus appears to be 

 similar to that of Penaeus, in that the two groups 

 of muscles are distinct. In Pandalus, on the other 

 hand, these muscle groups are so intertwined that 

 Berkeley refers to the lateral and medial parts as 

 slips of the same muscle. 



In Penaeus the dorsolateral and dorsomedial 

 abdominal muscles are markedly segmental and 

 readily separable from one another, except for the 

 dorsal abdominal muscle of the second segment 

 (occupying the dorsum of the first abdominal seg- 

 ment (fig. 62)). Here, the muscles are fused. 

 The medial portion is connected with the dorsal 

 thoracoabdominal muscle and a lateral slip of 

 fibers is attached to superficial connective tissue. 

 The whole structure passes caudad to insert on the 

 dorsal apodemal yoke dividing the first and sec- 

 ond somites. The dorsolateral muscles occupying 

 the second, third, fourth, fifth, and sixth abdomi- 

 nal segments all arise from the cuticular yokes 



