CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 22/ 



comparatively small, and while they are connected with the pos- 

 terior pallial arteries, the size and character of the vessels indi- 

 cates that the junction is probably very near the anterior ear. 



Considering everything, it seems likely that the longitudinal 

 axis of the body could be morphologically represented by a line 

 drawn from near the hinge extremity of the anterior ear to the 

 middle of the adductor muscle, and that a very small portion of 

 the scallop is anterior. 



CIRCULATORY SYSTEM. 



The animal is large enough to allow one to successfully inject 

 the chief vessels with starch or gelatin injecting mass, and then 

 by dissection and microscopic preparations to trace the dis- 

 tribution of the vessels of the different organs and to determine 

 quite definitely the course taken by the blood in its circulation. 



The heart is a typical, symmetrical lamellibranch heart, with 

 two auricles and one ventricle (Figs. I and 3), the latter per- 

 forated by the intestine which enters it near one end and leaves 

 it near the other end (Fig. 2). Dorsally the ventricle is prolonged 

 somewhat, posterior to the intestine, where the morphologically 

 anterior aorta is given off, and ventrally to a less extent it is 

 prolonged anterior to the intestine, where the much smaller 

 morphologically posterior aorta is given off. The walls of the 

 ventricle are of about even thickness throughout their extent, 

 and are quite smooth outside and inside. The auricles join the 

 ventricle on each side near its middle, are somewhat triangular 

 in shape, with the most acute angle receiving blood from the 

 gills and mantle at a point dorsal to the adductor muscle and 

 directly ventral to, but some distance from, the cartilage. The 

 opening of each auricle into the ventricle is near the middle of 

 the side of the auricle that lies next to the ventricle and farthest 

 away from the opening where the auricle receives its blood. 

 The muscles around the openings of the auricles into the ven- 

 tricle, and to a less extent around the openings through which 

 the auricles receive blood, are well developed and must act as 

 sphincters that tend to keep the blood from being regurgitated 

 The walls of the auricles, unlike those of the ventricles, are 

 roughened by pits that open into the cavities of the auricles. 



