1883.] NATURAL SCIENCES OF PHILADELPHIA. 221 



beneath, behind and, in A. Jluviatilis, on the right side; the left 

 side being covered by the albuminous gland. In A. lacustris the 

 relation is only reversed, so that the liver lies on the left side of 

 the stomach and the albuminous gland on the right. 



The intestine passes from the stomach at about the middle of 

 its superior wall and then passes into the liver, forming a loop, 

 which is clearly visible when the shell is removed in A.Jlaviatilis, 

 but difficult to be seen in A. lacustris. After a few turns it passes 

 to the left side of A.Jiumatilis and to the right in A. lucustris^ 

 and proceeds downwards, entering the gill and opening on the 

 external surface of the same. 



I will here call attention to a peculiar ring of long cylindrical 

 epithelial cells which lies in the walls of the rectum in A.fluviatilis. 

 It is in the middle of that part of the rec turn which lies in the gill. 

 These cells are ciliated, as are indeed the epithelial cells of the 

 whole intestine. 



The physiological significance of the cells forming the ring I 

 have in no way been able to determine. 



In both species the liver is large and fills up the greater part of 

 the bod3'-cavity. It consists of a number of follicles ; each follicle 

 is formed of an external tunica propria and an internal layer of 

 large cells. These cells secrete the bile, which is led into the 

 intestine, close behind its exit from the stomach, by means of 

 three ciliated ducts. 



The vascular system. — As the vascular system of Ancylus 

 differs so little from that of moUusca in general, it is not neces- 

 sary to go into details. The heart, which is an arterial one, is 

 formed of two parts, an auricle and a ventricle. In A. Jluviatilis 

 it lies on tiie left side of the body above the gill and in advance 

 of thei rectum. The auricle, the smaller of the two parts, is 

 divided from the ventricle by a contraction, and at this point a 

 valve is found opening into the ventricle. From the end of the 

 ventricle arises the aorta, which soon divides into two branches ; 

 one of these passes to the head {Arteria cephalica) and the other 

 supplies the viscera. These two branches divide into siTialler ones, 

 and finally open into the body-cavity, where they pour out their 

 blood. The blood, which can freely circulate in this cavity, is 

 collected into the lacunae of the foot which open in the floor of 

 the body-cavity. One of these lacunae, which can almost receive 

 the name of vein, passes from the foot into the mantle and becomes 



