NO. 9 



MUSCULATURE OF THE BLUE CRAB COCHRAN 



17 



many of the blue crab's appendages might be expected to show a varia- 

 tion from the usual structure, and this expectation is fulfilled when the 

 mandible is examined and compared specifically to that of the crayfish 

 and shrimp. Because of its two anterior articulations, to which ref- 

 erence has already been made, the mandible of the blue crab lies in a 

 partly reversed position ; as a matter of fact, its true anterior border 

 now is its upper posterior border when the crab occupies a normal 

 attitude, and its true posterior surface is now entirely ventral in 

 position. 



The primitive appendage, as shown by R. E. Snodgrass in his " Evo- 

 lution of the Insect Head and the Organs of Feeding," ° has essentially 



Fig. 6. — Diagram of the theoretical elementary musculature of the segmental 

 appendages (after Snodgrass). 



a-b, primitive dorsoventral axis of the appendage. 



/, dorsal promoter muscle ; /, dorsal remoter ; K, ventral promoter ; L, ventral 

 remoter; T, tergum ; Sfn, sternum; Appd, appendage. (After R. E. Snodgrass, 

 " The Thoracic Mechanism of a Grasshopper and its Antecedents," Smithsonian 

 Misc. Coll., vol. 82, no. 2, p. 10, 1929.) 



four muscles to control the movements of its basal part, two of which 

 originate in the dorsal region of the body, and two on the ventral 

 region (see fig. 6). The dorsal muscle, which is inserted on the ante- 

 rior upper border of the rim of the appendage, is called the dorsal 

 promotor (/), and the corresponding muscle inserted on the posterior 

 upper border is the dorsal remotor (/). The muscle inserted on 

 the anterior lower rim of the appendage is the ventral promotor {K), 

 and the corresponding muscle with a posterior lower insertion is the 

 ventral remotor (L). 



An attempt has been made (fig. 5 B) to analyze the extrinsic muscles 

 of the mandible in the blue crab to see just how they conform to the 

 simple ancestral type. It was found that the dorsal muscle numbered 



* Smithsonian Rep. 1931, p. 465, fig. 14, 1932. 



