MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 181 



TIk' left posterior coluniellar artery runs forward and upward to the dorsal edge of the left 

 shell muscle near its attachment. After giving off a branch to the dorsal portion of the body 

 wall it liends downward along- the inner surface of the shell nmscle, giving oti' numerous branches 

 which enter the muscle. 



The right posterior columellar artery arises from the right side of the aorta about o millime- 

 ters anterior to the origin of the hepatico-columellar artery. Since the posterior portion of the 

 aorta is upon the left side of the body cavity, the right posterior columellar artery is considerably 

 longer than the corresponding left artery. The cour.se of the right posterior columella)- artery is 

 the same, only inverted, as that of the left. 



About 20 millimeters anterior to the origin of the last artery the right pallio-nuchal artery 

 arises as a branch of the aorta. The left pallio-nuchal artery arises from the aorta 4 or 5 milli- 

 meters anterior to the right pallio-nuchal. These arteries pass upward and outward to the dorsal 

 side of the body, entering the body wall at the base of the mantle fold where the latter crosses 

 the dorsal edges of the shell muscles. Several branches leave the pallio-nuchal arteries at this 

 point. Some of these pass into the dorsal portion of the mantle lying against the involution of 

 the shell. The main portion of each artery is continued in the thin wall of the dorsal nuchal 

 region, supplying especially the cresceiitic fold upon the posterior face of the hood. As it 

 passes along the edge of the shell muscle it appears to send some small branches into the ti.ssues 

 of the muscle. A considerable branch extends into the crus of the funnel. From the outer side 

 of the pallio-nuchal artery a branch enters the mantle, which becomes continuous with the 

 marginal pallial branch of the anterior pallial artery. 



Usually no other vessels arise from the aorta until it divides into tiie innominate arteries. 

 The anterior proventriculai- arteries, supplying blood to rather more than the anterior half 

 of the proventriculus, frequently arise from the junction of the aorta with the innominate 

 arteries. These arteries are, however, extremely variable in their position, a fact to which 

 WiLi-EY has called attention. One or both may arise frou) the innominates, or one may be 

 entirely absent. In Willey's Fig. 28 (ISitti, 1), the left anterior proventricular artery arises from 

 the aorta a considerable distance l)elow its division into the innominate arteries. Two small 

 arterioles going from the anterior proventricular arteries to the walls of the aorta show also 

 considerable variation in their points of origin. Ordinarily one arises from the l)ase of each 

 proventricular artery. In the case iigured by Willey both arterioles arise from the right 

 anterior proventricular arteiy, the left proventricular artery being absent in this case. 



The buccal artery usually springs from the right innominate close to its separation from the 

 left. It passes forward upon the dorsal side of the buccal mass, presently dividing into three 

 branches. The median branch, the superior mandibular artery, runs straight forward in the 

 median line of the buccal mass, giving oti' branches to the superior mandibular nuiscles. The 

 lateral branches first pass outward to the sides of the ])uccal mass, giving off on the way several 

 small })i-anches posteriorly to the mandibular muscles, then turn forward and pass into the 

 buccal membrane and its papilhv. Of the origin of the tniccal artery Willey says: "It is a 

 singular fact that the gi-eat median buccal artery always springs from the right innominate artery. 

 The constancy of this origin would seem to indicate that it is potentially a paired structure." I 

 have dissected specimens in which the buccal artei-y s])rung from the junction of the innominate 

 arteries; in other words, was median. Either position, lateral or median, may be .secondary, 

 resulting from a displacement of the ba.se of the artery during growth, and it is difficult to decide 

 which is primitive without the evidence of embryology. 



Five or six millimeters from their junction a branch arises from the anterior side of each 

 innominate (the inferior mandibular artery), which runs forward on the under side of the ])uccal 

 mass to the nuisdes and organs of the fioor of the pharynx. These arteries are closely bound 

 to the buccal nervous S3'stem, careful dissection being required to separate the nervous from the 

 arterial elements. The inferior mandibular arteries supply not only the lower parts of the 

 mandibular muscles but also the tongue and the radular sac, the processes anterior to the tongue, 

 and the salivary processes. 



