286 



COLLEGE ZOOLOGY 



testine and return to the tissue spaces. The 

 large digestive gland produces a digestive 

 secretion containing the enzyme amylase; 

 this secretion passes through ducts that 

 empty into the stomach. 



In one family (Unionidae) of the fresh- 

 water clams, there is a carbohydrate-digest- 

 ing enzyme set free in the stomach by dis- 

 solution of a gelatinous rod (crystalline 

 style) which lies in a pouch off the intes- 

 tine and projects into the stomach. 



Circulation 



The circulatory system consists of a dorsal 

 heart, blood vessels, and spaces called 



sinuses. The heart lies in the pericardium 



(Fig. 175). The ventricle drives the blood 

 forward through the anterior aorta and back- 

 ward through the posterior aorta. Part of 

 the blood passes into the mantle, where it is 

 oxygenated, and then returns directly to the 

 heart. The rest of the blood circulates 

 through numerous spaces in the body and is 

 finally collected by a vein, which lies just 

 beneath the pericardium. From here the 

 blood passes into the kidneys, then into the 

 gills, and finally through the auricles into 

 the ventricle. Nutriment and oxygen are 

 carried by the blood to all parts of the body, 

 and carbon dioxide is disposed of in the 



Hinge ligament- 



Pericardial cavity- 

 Auricle 



Suprabranchial 

 chamber 



Cerebroviscerol 

 connective 



intestine- 



^-^'-°'^ °' cn^^^Kyw 



Intestine- 



-wmim^i 



mB I^M I \i foi fi 



Ventricle 

 Rectum 



Nephridium 



i 



Gill- 

 Foot- 



Gill- 



■Gonad 



Water tube of gill 



Mantle cavity 



Mantle 



Shell 



Figure 175. Cross-section of a fresh-water clam through the region of the heart. 



mantle and gills; other waste products of 

 metabolism are transported to the kidneys. 



Respiration and excretion 



Although the entire body surface is in 

 contact with water and doubtless functions 



in respiration, the greater part of the oxy- 

 gen-carbon dioxide exchange occurs in the 

 gills and mantle. A pair of gills hang down 

 into the mantle cavity on either side of the 

 foot (Fig. 175). Each gill consists of two 

 plates or lamellae made up of a large number 



