136 MOLLUSCA. 



posterior end, where it bends abruptly to the right along the 

 margin of the pericardial cavity, and enters the auricle. The 

 other returns blood from the tubuliferous portion of the kidney 

 and follows the right side of the pericardium to the auricle. 



3. The gill receives its blood through a vessel that borders 

 its right side. This vessel receives the blood from a portion of 

 the mantle, and from the large, acinous portion of the kidney. 



4. The blood leaves the ventricle by a single vessel, the aorta, 

 that almost immediately gives rise to the visceral artery which 

 supplies the visceral hump. Trace its distribution. 



The aorta makes an abrupt turn downward and forward and 

 enlarges to form the secondary heart which lies alongside the eso- 

 phagus. Follow the course of the aorta and its branches. 



The course of general circulation is, beginning with the heart, 

 (a) system, (b) kidney, (c) gill, and (d) back again to the 

 heart. What is the advantage of such a course of circulation 

 over the reverse? 



Draw a figure showing the vascular system. 



Excretory System. — The two portions of the kidney have 

 already been noticed. Cut along their common line of union 

 and examine the inner surface of each part. 



1. Notice the parallel lines of tubules that form the sub- 

 stance of the tubuliferous portion, and the lobules that form the 

 comparatively thick walls of the acinous portion. 



2. Find the slit-like opening that leads from the kidney to 

 the mantle cavity. It is at a point between the two portions of 

 the kidney and is easily found from the mantle chamber. A 

 small opening leads into the pericardium, but it is hard to find 

 it in dissections. 



Digestive System. — 1. Remove part of the integument at the 

 base of the proboscis and find the muscles that retract it. How 

 many are there and how are they attached? Do you understand 

 how the proboscis is extended? 



2. With a pair of scissors open the extended proboscis along 

 the ventral line, pin it open, and notice that the exposed muscu- 

 lar mass, the buccal mass, is attached to the wall of the proboscis 



