A FRESH-WATER CLAM 77 



Exercise 6. Draw a diagram of the respiratory system showing the 

 gills and their relation to the suprabranchial passages. Show the 

 direction of the flow of the respiratory water by means of arrows. 



Exercise 7. Draw a diagram showing the minute structure of a 

 lamella. 



The Circulatory System. With fine scissors and great care cut 

 open the pericardium by a slit along its dorsal border. Note the 

 heart, with the rectum passing through it. The heart consists of 

 three chambers : a median, thick-walled ventricle and two lat- 

 eral auricles. These latter are delicate, thin-walled organs, tri- 

 angular in shape, the base of the triangle lying along the dorsal 

 border of the gills and the apex communicating with the ven- 

 tricle. If the left auricle has been injured in the dissection, the 

 right one is easily seen by looking across the pericardial space. 

 From the ventricle an anterior and a posterior artery pass to 

 either end of the body. These arteries lie alongside the rectum, 

 to which the anterior one is dorsal and the posterior one is ven- 

 tral ; they are difficult to distinguish from it, except in speci- 

 mens 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 ; on its return 

 course it is first 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 interfilamentary connections, and is oxygenated; it then 

 passes into the auricles. 



The Excretory System. This system consists of a pair of kid- 

 neys, which are dark-colored organs lying just beneath the peri- 

 cardium 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 lies 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 on the side of the visceral mass beneath the an- 

 terior end of the kidney and near the base of the inner gill. It 

 is usually difficult to find but may be recognized by its white 

 lips. The kidney also possesses at its anterior end a duct lead- 



