NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN I 5 



jp. Muscnlus prodnctor flagellaris II antennae (fig. 4) . — Arising on 

 the proximal posterior wall of the meropodite, this muscle is inserted 

 on the base of the first annulus of the flagellum, which is pulled out- 

 ward and backward by its contraction. 



40. Miiscidus rcdnctor flagellaris II antennae (fig. 4) . — This muscle 

 arises on the anterior wall of the meropodite and is inserted on the 

 anterior part of the first ring of the flagellum, causing the latter to be 

 brought inward and forward. 



THE MANDIBLE 



As in the crayfish, shrimp, and lobster, the mandible in the blue crab 

 is firmly fixed at two articulations {x and xx, fig. 5) and hence cannot 

 rotate. 



The position of these articulations, however, is quite different in 

 the blue crab from that of corresponding articulations in the crayfish 

 and its allies, and a difi^erent mechanism for controlling the mandible 

 is required. In the crayfish, shrimp, and lobster, one of the articula- 

 tions is at the extreme upper anterior corner of the mandible, and the 

 other is at the lower posterior corner. Therefore any muscles connect- 

 ing the lower anterior corner with the skeletal part near the midline 

 will pull the lower halves of the mandibles strongly together, function- 

 ing thus as adductors. A muscle attached to the upper posterior edge 

 of the mandible, and running from the same central skeletal founda- 

 tion, perhaps beside and even parallel to the adductors just described, 

 will pull the mandibles just as strongly apart, performing the function 

 of abductors. This opposition is made possible by the widely separated 

 points of articulation of the mandible, which allow its upper and lower 

 borders to pivot inward and outward between their hinges. This 

 swinging motion is further intensified by such additional abductors and 

 adductors as give sufficient power to the masticatory function of the 

 mandible. 



In the blue crab the articulations of mandible with head skeleton are 

 both anterior, one at the upper and one at the lower corner of the 

 mandible. Because of these anterior articulations, any muscles going 

 from the central foundation to any available spot on the inner poste- 

 rior surface of the mandible behind these forward-lying hinges are 

 bound to open the mandible, functioning as abductors. Hence there is 

 no anterior adductor in the blue crab, and the thin sheetlike muscle of 

 the blue crab, which corresponds to that muscle in the crayfish, func- 

 tions now as a major abductor of the mandible, and all the work of 

 closing the mandible has to be done by the very heavy and powerful 

 posterior and lateral adductors. 



