WHITE SHRIMP FROM THE GULF OF MEXICO 



111 



passing across the anterior part of each abdominal 

 segment and from them run caudad to the next 

 yoke. The axis of the fibers of the lateral muscles 

 is approximately parallel with the long axis of the 

 animal. 



The fibers of the dorsomedial abdominal mus- 

 cles are oblique to the long axis of the white 

 shrimp. These muscles lie beneath the dorsolat- 

 eral muscles. Each originates on a median sagit- 

 tal apodeme within a segment and runs laterad to 

 insert on an intersegmental apodeme, posteriorly. 

 The arrangement in Penaeus is far simpler than 

 that of the dorsal abdominal musculature of 

 Pandalus. 



Dorsad of the muscles under discussion, and 

 very close to the middorsal line, are paired struc- 

 tures passing rostrad from the cuticular tissue di- 

 viding the abdominal segments (see lower half of 

 fig. 62). The structures appear to be flattened 

 tubes, soft and easily broken. They resemble ves- 

 sels of the circulatory system more than anything 

 else. Their function is unknown. 



MAIN VENTRAL ABDOMINAL MUSCLES 



The illustrations show that relatively few ab- 

 dominal muscle types exist. However, the few 

 that do are serially repeated and so extensively 

 interwoven that their isolation is extremely diffi- 

 cult. Consequently no one should be surprised that 

 an understanding of abdominal muscle function is 

 of fairly recent derivation. Much of what we 

 know is based upon the work of Daniel (1928, 

 1929, 1931a, 1931b, 1931c, 1931d, 1933) who first set 

 forth the details of the comparative anatomy of 

 abdominal muscles in higher Crustacea. 



One of the most interesting features of abdomi- 

 nal anatomy is the contrast between the lightly 

 sclerotized abdominal skeleton and the very 

 heavy abdominal musculature. One would almost 

 expect the light cuticle to be damaged by the 

 powerful flexions of the abdomen. That such 

 does not occur is explained by the unusual arrange- 

 ment of the abdominal muscles. Some of them are 

 attached to prevent extreme distortions that might 

 otherwise take place. Some muscles, in a sense 

 then, substitute for the skeleton. In addition, cer- 

 tain muscles act in the functional sense as great 

 apodemes of other muscles, thereby freeing the 

 abdominal skeleton of the requirement to produce 

 all but the simplest apodemal material. These 

 apodemal muscles, furthermore, act as fulcra to 



improve the mechanical advantage of long mus- 

 cles, such as the anterior obliques, connected be- 

 tween two widely separated abdominal segments. 

 The terminology of the abdominal musculature 

 of Penaeus has been adopted from the work of 

 Daniel. For this reason, substantial differences in 

 nomenclature will be found between the present 

 study and the older morphological research. Dan- 

 iel ( 1931c, 1932) has renamed some abdominal mus- 

 cles and subdivided others. For the most part. 

 however, he makes use of the classical abdominal 

 muscle names. 



CENTRAL MUSCLES OF ABDOMEN 

 Figure 61 



The longitudinal central muscles are among the 

 main fulcral muscles of the abdomen. They lie 

 deep within the abdominal muscle mass. Six pairs 

 of central muscles are found in the abdomen of 

 Penaeus, joined together end to end in a wavy 

 chain. Each central muscle takes origin on an 

 apodeme of a dorsal branch of the preceding cen- 

 tral muscle. These dorsal apodemes lie above the 

 transverse muscles. As the central muscle passes 

 caudad into the succeeding segment, it is first 

 joined from above by the fibers of the oblique 

 transverse muscle and then, in the neighborhood 

 of the succeeding transverse muscle, the central 

 muscle divides. The dorsal slip joins the apodeme 

 of the next central muscle and the larger ventral 

 part turns mesad along the anterior face of the 

 transverse muscle. The central muscle apodemes 

 dorsad of the transverse muscles also give rise to 

 the posterior oblique muscles. 



The counterparts of the central muscles of 

 Penaeus in Astacus are not easy to determine. The 

 only muscles in the European crawfish correspond- 

 ing to the central muscles are the musculi ventrales 

 profundi of Schmidt. The central muscles func- 

 tion to support the oblique muscles during the con- 

 tractions of the latter. 



TRANSVERSE ABDOMINAL MUSCLES 



Figures 60. 61, 64 



The large transverse abdominal muscles attach 

 dorsolaterally on the abdominal tergum (fig. 60) 

 and run across the abdomen in association with 

 strong fasciae (the fasciae arise at points indi- 

 cated by small triangles in fig. 60) . The abdomen 

 of Penaeus contains six transverse muscles. A mid- 

 sagittal view of the abdomen (fig. 61) shows the 



