8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



The muscles of this pair converge shghtly before reaching their inser- 

 tions on a T-shaped infolding of the ventral part of the middle cylin- 

 der, in front of the attachment of the anterior basal muscle. As this 

 middle cylinder is cartilagelike and hence somewhat pliable, the attrac- 

 tor can assist the anterior basal muscle in depressing it and hence in 

 bringing the solid joints attached to it nearer together. It may likewise 

 oppose the basal muscle in rotating the second joint. 



iS. Miisculus ociili adductor (fig. 2). — This heavy and powerful 

 but short muscle arises on the thick membrane separating the ventral 

 part of the second joint from the head. It travels forward and outward 

 to its insertion along the anterior distal wall of the second segment not 

 far from the base of the optic cup, which is rotated strongly by its 

 contraction. 



ip a, h. Muscuhis oculi abductor a and b (fig. 2). — Originating 

 posteriorly on the heavy membrane which connects the second joint 

 to the optic cup, the main part (a) of this muscle is inserted on the 

 posterior wall of the optic cup near to the corneal surface. This is the 

 largest and heaviest of any of the muscles lying in the cup. The 

 second branch {b) originates beside the first but juts off at an angle 

 toward the ventral surface, where it is soon inserted not far from the 

 proximal border of the optic cup. It is much shorter than the main 

 branch, from which it is separated near its insertion by the lateral 

 retractor muscle. Both branches oppose the adductor by pulling the 

 eye away from the midline and rotating it in the opposite direction. 



THE RETRACTOR MUSCLES OF THE EYE 



Like the crayfish and shrimp, the blue crab possesses four retractor 

 muscles, all of which originate on the membrane bordering the distal 

 edge of the second segment and are inserted on the sides of the 

 optic cup near the cornea. They bring the cup nearer to the second 

 segment or rotate it. The insertion of each muscle is marked exter- 

 nally by a characteristically different texture in the surface of the 

 optic cup. 



20 o-c. Muscuhis ocidi retractor dorsalis a, b, and c (fig. 2). — This 

 muscle has three branches, all of which arise from a heavy ossiclelike 

 projection lying in the membrane and originating on the dorsal distal 

 wall of the second segment. The main branch, the central one of the 

 three, travels outward to its attachment on the dorsal surface of the 

 optic cup, where its insertion is marked externally by a small area of 

 a slightly granular texture different from the smooth surface around it. 

 The second branch {b) projects forward at right angles to the first 



