178 THE FRESH-WATER MUSSEL [CH. VI 



a long and careful series of experiments and observations 

 is still needed here; they should be conducted upon 

 specimens taken fresh from their native waters and not 

 upon individuals kept in aquaria. 



The food appears to pass very slowly along the 

 intestine for it may be found there after specimens have 

 been kept for several days in perfectly clear tap- water. 

 The ventral wall of the last section of the intestine, or 

 rectum, where it runs dorsally from the visceral portion 

 of the foot towards the pericardial cavity, is folded so 

 as to form a prominent ridge, the typhlosole, projecting 

 into the gut cavity. This fold increases the internal 

 absorbing surface and narrows the channel along which 

 the food is passing, thereby possibly accentuating the 

 effect of the cilia upon the fluid contents. The faeces 

 are often expelled with considerable violence, being shot 

 forth a distance of six or eight inches. They appear 

 to me to travel in a small body of water moving as a 

 vortex ring, for though their own movement is rapid 

 there is no apparent disturbance of the surrounding water 

 which they traverse. I am unable to state by what 

 muscles this forcible defaecation is produced : no move- 

 ment of the shell-valves could be detected. 



Circulation, Respiration 1 . The heart, which consists 

 of a single ventricle wrapped round the rectum and a pair 

 of triangular auricles, is situated dorsally close beneath 

 the mantle. It lies in the pericardial space which it 

 practically fills during life. The pulsations can be plainly 



1 Langer, Denkschr. Wien. Akad. vin. Abth. 2; Willem. op. cit. ; 

 Owsjannikow, Bull. Acad. Imp. Sci. St Petersb. (5) n. 



