CH. VI] THE FRESH-WATER MUSSEL 179 



seen through the thin overlying structures after removal 

 of the animal from the shell ; their rate is from four to six 

 per minute but it varies with the temperature and with the 

 intraventricular blood-pressure. Two main arteries, one 

 from each end of the ventricle, convey oxygenated blood 

 from the heart to all parts of the body. The anterior 

 aorta runs dorsal and the posterior ventral to the rectum. 

 After traversing the lacunar spaces of the foot and viscera 

 the blood is returned to a large median vena cava on the 

 ventral side of the pericardium ; hence it flows along the 

 two afferent branchial vessels through the nephridia to 

 the gills, where it receives fresh supplies of oxygen and 

 parts with its carbon dioxide; the efferent branchials 

 return the blood from the gills to the auricles, receiving 

 also oxygenated blood returning from the mantle lobes. 

 The last named portion of the blood does not pass through 

 the gills at all ; it is thus evident that the mantle lobes 

 are of equal importance with the gills as respiratory 

 organs. The auricles drive the blood into the ventricle 

 through valved apertures by which reflux is prevented. 

 The pressure within the auricles during their relaxation 

 is negative and thus blood is, as it were, sucked back 

 towards the heart. Valvular arrangements are present in 

 the form of sphincter muscles which can constrict the 

 cavity of the vessels in whose walls they are placed. Of 

 these the most important is " Keber's valve " already 

 mentioned in connexion with the turgescence of the 

 foot. 



The blood is colourless, and contains colourless 

 amoeboid corpuscles which take up and absorb degene- 



122 



