V.] THE FRESH-WATER MUSSEL. 325 



nature of the former : at its point of communication 

 with the auricle will be seen the auriculo-ventricular 

 valves; long pocket-shaped flaps, so disposed as to 

 admit only of a flow of the blood from the auricle 

 to the ventricle. 



2. Remove all but the bases of the auricles, and cut the 

 rectum across at its extreme anterior end; turn it 

 and the heart backwards, so as to lay bare the floor 

 of the pericardium. Running along the middle line 

 of this will be seen a large blood- sinus, the vena cava: 

 it lies between the two excretory-organs. 



a. At the extreme front end of the pericardiac floor, 

 immediately under the point at which the intestine 

 enters the cavity, will be found the reno-pericardial 

 apertures ; pass a bristle, or seeker, into one of 

 them and open up the excretory vestibule of the 

 same side. The glandular segment will now be 

 seen the seeker projecting into it. Examine the 

 general relations of the excretory organs, and 

 their apertures of communication with the supra- 

 branchial chamber. (Cf. Sect. E.) 



b. Examine the floor of one of the auricles under 

 a lens, first having washed the specimen quite 

 clean. Note the large orifices of the efferent bran- 

 chial vessels which open into it. 



c. The organ of Keber. Follow its cut edge; it em- 

 braces the antero-lateral region of the pericardium. 

 (Cf. Sect. B. e. a.} 



G. The circulatory system. 



i. The arterial system. Lay bare the pericardial cavity 

 and insert the point of a medicine-dropper, filled with 



