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. 



1. The arterial system. Lay bare the pericardial cavity 

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



