NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN 3 



PART 1. THE MUSCLES OF THE TRUNK AND ITS APPENDAGES 



The Trunk 



The complete fusion of the segments of head and body in the blue 

 crab has resulted in the disappearance of those intersegmental muscles 

 which in crustaceans like the shrimp and the crayfish give a high 

 degree of flexibility to the movements of the body. 



The crab's head and body are encased in a hard, unj (tinted covering, 

 which shows no trace whatever of segmentation on its dorsal sur- 

 face, although ventrally the sternal thoracic segments on which the 

 basal leg muscles originate are well marked. Of all the extremely 

 complex and numerous body muscles that one encounters in the shrimp 

 and crayfish, there is but one, the attractor of the epimera, which finds 

 a counterpart in the blue crab, where it performs the same function 

 of holding the gill chamber in its proper relation to the carapace. 



While the abdomen of the crayfish and shrimp is extremely pliable 

 and is much used in swimming, the abdomen of the blue crab, in 

 the male at least, is apparently progressing toward a condition of par- 

 tial rigidity, as the third, fourth, and fifth segments are immovably 

 fused in that sex. This fusion is not yet completely established, how- 

 ever, as the former segmentation is still partly maintained in its mus- 

 culature. The female's abdomen has six distinct segments, all of 

 which have the muscles well developed. The structure of the hard 

 parts of the abdomen of the male is such that it can not be extended 

 behind the body in line with the back, but at most can assume a position 

 at right angles to the dorsal surface of the body. The abdomen in both 

 sexes normally lies closely adpressed against the posterior region of 

 the thorax. In this position, the dorsal part of the abdomen is under- 

 neath the body and actually ventral in position. In the text, how- 

 ever, it is described by the term " dorsal," applied to that part which 

 would be uppermost in a normal crustacean abdomen extending back- 

 ward behind the thorax. 



I. Mitscitliis veiitralis super ficialis thoraco-abdoiiiiiialis (fig. i B). — 

 This muscle arises on the outer posterior surface of the last segment 

 of the thorax and is inserted on the anterior border of the first abdomi- 

 nal segment near the midline, where it helps to pull the abdomen 

 toward the thorax. This is the only trace in the blue crab of the 

 ventral superficial thoracic muscles, which are so prominent between 

 the highly movable body segments in both Asiacus and Paiidalus.^ 



^ In the particular discussion of the muscles, the comparisons made to homolo- 

 gous parts in the shrimp and crayfish refer only to the species Pandalus danae 

 and Astaciis fluviatilis. 



