54 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



species of Penaeus, Caiman (1909) describes a 

 mastigobranchia on the first maxilliped and 1 ar- 

 throbraneh and 1 pleurobranch on the second 

 maxilliped. Otherwise the branchial formulae of 

 the two penaeids are the same. 



MUSCLE ELEMENTS 



Of the numerous muscles attaching throughout 

 the gnathothoracic region, many belong to the 

 mouthpart and thoracic appendages and will be 

 taken up when the latter are discussed. Others are 

 protocephalon muscles and have already been con- 

 sidered. Still others are associated with the ali- 

 mentary canal and heart and will be dealt with in 

 the sections concerned with these organ systems. 

 The many remaining muscles are either small su- 

 perficial lateral and ventral muscles of the thorax, 

 or large dorsal and ventral muscles. Some of these 

 are morphologically thoracic muscles, and some 

 are morphologically abdominal muscles. In the 

 functional sense, the foregoing muscles may be 

 classified as abdominal musculature, for all of the 

 large muscles taking origin on extensive areas of 

 the dorsal and ventral thoracic skeleton represent 

 the major muscular attachments of the abdomen 

 to the thorax. Substantial movements, espe- 

 cially in the dorsoventral plane, are possible be- 

 tween the thorax and abdomen and it is these 

 muscles which mediate the movements. 



SUPERFICIAL LATERAL THORACIC MUSCLES 



Figure 33 



Stretching between the dorsal reaches of the 

 pleural brachia are at least five superficial lateral 

 thoracic muscles. These muscles are extremely 

 thin and weak. Apparently they give rigidity to 

 the thin cuticle of the area during lateral move- 

 ments of the shrimp along the anteroposterior 

 axis. Schmidt (1915) and Berkeley (1928) do 

 not describe these muscles in Astacus and Pmida- 

 Iuk. The possibility that the epimeral attractor 

 muscles in Astacus and Pandalus are the super- 

 ficial lateral thoracic muscles of Penaeus is slight. 



VENTRAL MUSCLES 



SUPERFICIAL VENTRAL THORACIC MUSCLES 



Figure 36 



The superficial ventral thoracic muscles (fig. 

 36) are situated slightly laterad of the ventral 

 nerve cord. Their median parts are also ventrad 



of the nerve cord and the ventral (subneural) 

 artery. These muscles are thin and fan shaped, 

 broad anteriorly and narrowing to posterior at- 

 tachments to thin connective tissue fasciae. In 

 Astacus the lateral and ventral pleurosternal apo- 

 demes fuse above the ventral nerve cord to pro- 

 duce the mesophragm of the endoskeletal system. 

 The superficial ventral thoracic muscles of the 

 crawfish attach to these mesophragms. Berkeley 

 (1928) finds that the ventral thoracic muscles in 

 Pandalus attach to the endophragmal para- 

 phragms, the lateral fusion product of the pleural 

 and sternal apodemes. As stated above, Penaeus 

 has neither paraphragm or mesophragm; how- 

 ever, one would expect the superficial ventral 

 muscles to attach to the little pleurosternal apo- 

 demes near the limb foramina. Contrary to such 

 expectations, the superficial ventral thoracic 

 muscles in the white shrimp attach to small apo- 

 demes on the pleurosternal brachia. The result is 

 to place these muscles ventrad of the nerve cord. 

 Their function of drawing the ventral thoracic 

 segments together is probably the same as in 

 Pandalus and Astacus. 



In Penaeus 7 pairs of superficial ventral tho- 

 racic muscles are evident compared to 6 for Asta- 

 cus and Pandalus. Schmidt (1915), however, 

 finds two superficial ventral thoracoabdominal 

 muscles in Astacus compared to one in Penaeus 

 and Pandalus. 



ANTERIOR THORACIC MUSCLES 

 Figures 33 to 36 



By far the largest ventral muscles of the tho- 

 rax are the lateral and median anterior thoracic 

 muscles (figs. 33 to 36). The anterior-most mus- 

 cle of these is the lateral anterior thoracic muscle 

 1 (figs. 33 to 36). This muscle originates by a 

 large, lateral oval in the region just posterior and 

 slightly dorsad of the hepatic spine. In dorsal 

 view (fig. 35) the muscle may be seen to run pos- 

 teroventrally to join the other anterior thoracic 

 muscles on the ventral surface of the thorax. In 

 this area all the anterior thoracic muscles are in- 

 terconnected to segmentally arranged fasciae. 



The other lateral anterior thoracic muscles 

 (figs. 34, 35), Nos. 2, 3, 4, and 5, may best be seen 

 in ventral view (fig. 36). These muscles take 

 origin from the ventrolateral fascia of each tho- 

 racic segment and run posteriorly into the ab- 

 dominal musculature with the other anterior tho- 

 racic muscles. 



