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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



the abdomen. The branchial slit is apparently for the 

 purpose of allowing the water which has passed through 

 the gills to pass back into the branchial chamber, and again 



FIG. 148. 



Fia. 148. Diagram of a vertical section of Unio purpurea, passing 

 through the heart. (Drawn from nature by "W. K. Brooks.) 



a to t. as in Fig. 146. w. Abdomen, r. Pericardium, w. Ventricle, 

 x. Auricles, y. Sinus venosus. z. Non-glandular portion of organ of 

 Bojanus. 



through the gills, so that the branchial current need not be 

 interrupted when the animal is out of water, with its valves 

 closed ; this arrangement is of importance in such marine 



