6 SMITHSOXIAN MISCELLANEOUS COLLECTIONS VOL. 92 



slender, to correspond to the shape of the male's ahdomen, and is in- 

 serted on cartilagelike outgrowths emanating from the anterior border 

 of the telson, which receives all its power of motion from this muscle, 

 as no flexors of the telson exist in either sex. The female's dorsal 

 superficial muscles are like those of the male, except that all six ab- 

 dominal segments are distinct and hence the full complement of six 

 pairs of muscles is present and functional. The dorsal muscles serve 

 to extend the abdomen backward, but as this position of the abdomen 

 is not habitual in the blue crab, occurring only at the time of mating, 

 the muscles are very weakly developed. 



14. Musculus attractor epimcralis (figs. 12 A, 13 B). — All that re- 

 mains of this muscle, extensive in both Astacus and Pandalus, is a small 

 patch of short muscle fibers uniting the epimeral plates and the cara- 

 pace, between the metabranchial and the cardiac regions. It extends 

 only for a short distance from the posterior angle of the first epimeral 

 plate. It holds the gill chamber in place in the body, beneath the 

 branchial lobe and the posterior part of the protogastric region, on 

 which the muscle originates. 



The Eye 



The eye of the blue crab is a highly complex organ, which presents 

 many specializations in its structure and musculature. The shortening 

 and broadening of the body contour have also been repeated in the 

 changes that have taken place in the eyes. The crayfish and shrimp, 

 both with elongate, narrow bodies, have the eyes close together on 

 short stalks, which project forward in front of the head. The blue 

 crab, on the other hand, has eyes which project on very long stalks 

 at right angles to the axis of the body. The middle cylinder (/ in 

 fig. 2), quite distinct and having its own muscles in the crayfish and 

 shrimp, is completely fused ' to the chitinous middle ring in the blue 

 crab, and the muscles of these parts, formerly separated, are now 

 forced to interlace in a very constricted area. The second segment, on 

 the contrary, is immensely elongated in the blue crab. Its proximal part 

 contains no muscles, but only a deep groove in which lie the blood- 

 vessels feeding the eye. Ventrally, this part of the segment is separated 

 from the head by a thin membrane. This membrane thickens consider- 

 ably toward its distal boundary, and on this membrane the adductor 

 muscle arises, which is not the case in either the crayfish or the 

 shrimp. The muscles arising on the distal border of the second seg- 

 ment, or on the heavy tendinous outgrowths from it, bear much the 



*The entire fused structure will hereafter be spoken of as the middle cylinder. 



