68 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



a rounded but equally prominent ossicle, which is arbitrarily called the 

 middle pleuropyloric. The posterior pleuropyloric ossicle seems to be 

 lacking in the blue crab. It is named but not figured by Pearson 

 (1908) in his study of Cancer pagurus (p. 103). (Figs. 25 B, 26.) 



MUSCLES OF THE ALIMENTARY SYSTEM 



For the sake of conformity with the writings of other authors, the 

 muscles of the alimentary system are discussed according to their ori- 

 gin, following the definition of Mocquard,* who recognized two sets of 

 muscles — first, the extrinsic, in which the points of origin are on some 

 part of the body skeletal system and which are inserted on ossicles 

 lying in the walls of the stomach, and second, the intrinsic, which are 

 attached at both ends to stomach ossicles or to thickened parts of the 

 stomach membrane. 



EXTRINSIC MUSCLES 



The following three sets of muscles help to work the gastric mill : 



igy. Musciilus gastricus anterior. One pair. — Each muscle of this 

 pair has its origin on the cervical membrane and extends backward, 

 gradually convergent toward the midline. They are attached side by 

 side on the inner anterior part of the pterocardiac ossicle (//). These 

 muscles are the most readily detected of any of the stomach muscles, 

 as their large size and dorsal position bring them conspicuously to 

 view as soon as the carapace is broken in that region. (Figs. 29, 30.) 



1^8. Musciiliis gastricus posterior mcsalis. One pair. — These 

 muscles arise from two small calcareous projections on the under side 

 of the carapace at the median part of the mesogastric region. There is 

 a distinct transverse indentation or channel on the outside of the 

 carapace, which indicates the position of attachment of these muscles, 

 as well as that of the external posterior gastric muscles and the dorsal 

 pyloric muscles to be mentioned later. The inner posterior gastric 

 muscles extend forward and downward to their attachment on the 

 pyloric ossicle {VII). These muscles are not so heavy as the anterior 

 gastric muscles (ip/) just described. (Figs. 29, 30.) 



/pp a and b. Miisculus gastricus posterior lateralis. Txvo pairs a 

 and b. — These muscles arise from the under side of the mesogastric 

 region in the outer part of the same channel which marks the origin 

 of the inner posterior gastric muscles {198) discussed above. They 

 extend downward and forward, converging as they go, and the median 



* Mocquard, F., Recherches anatomiques sur restomac des Crustaces podoph- 

 thalmaires. Ann. Sci. Nat., 6 ser., Zool., vol. 16, p. 238, 1883. 



