NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN 7 



same relations to one another as in the crayfish and shrimp. There 

 are two branches to the abductor and three to the dorsal retractor, the 

 result being that the blue crab has excellent control of its eye 

 movements. 



ift.Musculus ociili basalis anterior (fig. 2). — This muscle arises 

 medially on the epistome from a short, curved, movable rod, which 

 projects first at right angles from the center of the epistome and then 

 slopes downward and backward over the esophagus and enlarges to 

 a buttonlike knob. From this knoli the muscle runs dorsally and soon 

 divides into two short but relatively thick branches, which find attach- 

 ment side by side below the proximal edge of the chitinous middle 



19a 23b 23a 20b 



\ \ / / 



/ 

 22 19b 20c 



Fig. 2. — Dorsal dissection of the eye of the blue crab. On the right side the 

 deeper muscles are exposed. 



IS, musculus oculi basalis anterior; 16, musculus oculi basalis posterior; //, 

 musculus oculi attractor ; 7^, musculus oculi adductor ; iga and igb, musculus 

 oculi abductor ; 20a, 20b and 20c, musculus oculi retractor dorsalis ; 21, musculus 

 oculi retractor ventralis ; 22, musculus oculi retractor lateralis ; 23a and 23b, 

 musculus oculi retractor medialis. 



/, middle cylinder ; //, second segment ; ///, optic cup. 



cylinder which unites the optic peduncles. The distal part of each 

 peduncle, bearing the retina, is thereby moved forward in a horizontal 

 plane, so that the eyes are brought slightly nearer together. At the 

 same time the second joint may be rotated slightly. 



16. Musculus oculi basalis posterior (fig. 2). — This muscle arises 

 on the knoblike part of the supporting rod of the preceding muscle. 

 It runs unpaired dorsally for a short distance, closely adherent to the 

 dorsally directed part of the preceding. Then it divides into two very 

 fine but exceedingly strong branches which diverge slightly as they 

 continue dorsally between the branches of the anterior basal muscle 

 to their attachment on the frontal region of the carapace of the head, 

 where their presence is marked usually by two small indentations. 



//. Musculus oculi attractor (fig. 2). — This short compact muscle 

 arises on the head carapace near its junction with the middle cylinder. 



