SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 92 



Although this muscle is paired, as are all the other abdominal muscles, 

 the members of the pair are so closely crowded toward the middle 

 line that they appear as one median bundle of muscle fibers. 



2-6. Musculi ventrales super ficiales abdominis (fig. i B). — These 

 muscles are arranged regularly in accordance with the original seg- 

 mentation of the abdomen in the male, and the fusion of the third, 

 fourth, and fifth abdominal somites in this sex has evidently not 

 affected the ventral musculature at all, since the latter is similar in 



Fig. I. — Muscles of the abdomen of the male blue crab. 



A, dissection of the abdomen from the ventral side to show the dorsal muscles : 

 7&, small branch of musculus dilatator ani ; 6'-/.?, musculi dorsales superficiales 

 abdominis. 



B, dissection of the abdomen from the dorsal side to show the ventral muscles : 

 J, musculus ventralis superficialis thoraco-abdominis ; 2-6, musculi ventrales 

 superficiales abdominis ; 7a, main branch of musculus dilatator ani. 



I-VI, abdominal somites i through 6; Tn, telson. 



both sexes. The muscles of the first pair (_=>) arise on the membrane 

 of the anterior border of the first segment and are inserted on the 

 heavy sclerotized ridge marking the second segment. Each muscle of 

 the pair splits into several diverging branches, the twO' inner ones 

 being practically confluent on the midline. The second (j) and third 



(4) pairs are similar to the first. Each muscle of the fourth (5) is 

 definitely in a single piece, however, and its posterior attachment is 

 made upon an arrow-shaped cartilagelike thickening of the membrane 

 in the middle of the segment. The muscles of the fifth and last pair 



(d) are likewise undivided, the two muscles lying very close together 



