Qo PHYLUM MOLLUSCA 



along the dorsal border of the gills and the apex communicating 

 with the ventricle. If the left auricle has been injured in the dis- 

 section, the right one is easily seen by looking across the pericar- 

 dial space. From the ventricle an interior and a posterior artery 

 pass to either end of the body. The posterior artery expands, 

 near the posterior end of the pericardium, to form a large, thick- 

 walled sac, the arterial bulb. These two arteries lie alongside 

 the rectum, to which the anterior one is dorsal and the posterior 

 one is ventral ; they are difficult to distinguish from it, except in 

 specimens in which the heart has been injected. 



The course of the blood is the following : by the contraction of 

 the heart the blood is sent to all parts of the body, whence it is 

 conveyed through a system of lacunae to the kidneys and thence 

 to the gills ; here it circulates in vessels which run through the 

 interlamellar partitions, the gill filaments, and the interfilamen- 

 tary connections, and is purified ; it then passes into the auricles. 



The Excretory System. This consists of a pair of kidneys which 

 lie just beneath the pericardium and in front of the posterior 

 adductor muscle. Each kidney consists of two parts, the kidney 

 proper and the ureter. The former is a dark, thick-walled gland 

 which Hes beneath the ureter and communicates with it at its 

 hinder end. The ureter is a thin-walled vessel lying above the 

 kidney proper, with a small external opening in the side of the 

 visceral mass near the base of the inner gill which is usually diffi- 

 cult to find. The kidney also possesses at its anterior end a duct 

 leading into the pericardial cavity. Slit open the ureter and kid- 

 ney proper and observe their inner structure. 



Exercise 8. Draw a diagram representing the pericardial cavity and 

 the kidney, showing the relation of the two structures to each 

 other. Draw the heart in the pericardial cavity, showing the rela- 

 tion of the auricle to the gills. 



The Digestive System. The stomach and intestine are em- 

 bedded in the visceral mass and are difficult to dissect out of 

 it. With care and patience, however, it can be done. Find the 

 mouth between its two pairs of palps; note the upper and 

 the lower lips, which connect the upper and the lower pair of 

 palps respectively. The mouth is seen to the greatest advantage 



