NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN 9 



and is attached on the front wall of the optic cup near its proximal 

 border. The third branch (c) projects also at right angles but in an 

 opposite direction to b, and is attached to the posterior wall of the 

 optic cup near its proximal edge. The three branches taken together 

 with the ossiclelike piece from which they originate form a cross, and 

 the attachment at the extremities of the cross produces a mechanical 

 device of great strength for moving the optic cup dorsally and for 

 rotating it from side to side. 



21. Muscuhis oculi retractor ventralis (fig. 2). — This is a relatively 

 small and weak muscle, which arises ventrally in the membrane emanat- 

 ing from the distal edge of the second segment and is inserted on the 

 ventral wall midway to the cornea. Since it runs parallel with the 

 axis of the eye, it cannot act as a rotator. Its only function is to retract 

 the optic cup. 



22. Musculus oculi retractor lateralis (fig. 2). — This muscle origi- 

 nates in a tendinous structure in the membrane of the posterior ventral 

 wall of the second segment, and passes diagonally backward and up- 

 ward between the two parts of the abductor to its insertion on the pos- 

 terior wall of the optic cup just above the insertion of the shorter 

 branch of the abductor. It has a strong rotatory function, owing to 

 its position diagonal to the axis of the eye. 



2^ a, b. Musculus oculi retractor medialis a and b (fig. 2). — This 

 muscle has two branches, both of which arise from an exceedingly 

 heavy ossiclelike projection from the anterior distal wall of the second 

 segment. The upper branch (a) proceeds straight along the anterior 

 wall of the optic cup to its attachment not far from the cornea. The 

 lower branch (b) diverges slightly downward to its attachment on the 

 anteroventral wall of the optic cup not far forward of the insertion 

 of the ventral rotator. The medial retractor has the rotatory function 

 in addition to being a retractor, as its diverging branches testify. 



The Appendages 



The problem of choosing names for the various muscles governing 

 the appendages has proved to be a very puzzling one, especially in 

 regard to those muscles governing the mandible, the maxillae, and 

 the maxillipeds. It is often impossible in the living crab to assign to a 

 definite one of the many complex muscles surrounding the base of 

 each appendage a particular motion observed in that part of the ap- 

 pendage. In the telopodite the case is much simpler, as there are but 

 two muscles governing each segment, and but two corresponding direc- 

 tions of motion. In the dissected crab, the many slender muscles con- 



