FRESHWATER MUSSELS 65 



mentary canal by cilia. The anus is situated on a conspicuous 

 spout a little anterior to the exhalant siphon. The position of 

 the excretory organs (nephridia) has a] ready been mentioned. 

 Their external openings lie, one on each side, just anterior to 

 the free dorsal edge of the inner sheet of the internal gill. The 

 aperture is very small, but has yellow, rather swollen lips by 

 which it may be distinguished on the otherwise dark external 

 nephridial surface. The internal openings of the two nephridia 

 are in the ventral anterior region of the space surrounding the 

 heart (pericardium). 



The heart itself lies inside the clear transparent walls of the 

 body, a little way in front of the posterior adductor muscle and 

 in the dorsal region. Its central part is yellowish and muscular, 

 and appears to be pierced by the last portion of the intestine ; 

 this part, the ventricle, is the force-pump which drives the blood 

 all through the body. On each side of the ventricle is a very 

 thin-walled, triangular auricle, the apex being attached to the 

 ventricle. The auricles receive blood from the large veins which 

 return the blood from the body, and pass it on to the ven- 

 tricle. If a living animal be removed from its shell the beating 

 of the heart can be seen, and will continue for many hours. 

 The rate of the beat is about five per minute, but is very 

 variable. 



Mussels have but few sense organs. They possess no eyes, 

 but the tentacles which surround the inhalant siphon can ap- 

 preciate the difference between light and darkness ; they are 

 also exceedingly sensitive to touch, and to the quality of the 

 water, which they in some way taste. The surface of the foot is 

 also sensitive to light, but no special optic organs have been 

 discovered. 



Reproduction and Life-History. The sexes of mussels are 

 separate, but there is no external distinction between the males 

 and females. The genital glands in either case lie in the dorsal 

 part of the " foot/' and their openings are close to those of the 

 nephridia, being slightly ventral and posterior to them, but 

 very difficult to detect. The spawning season is in May and 

 June with Unio, but later, extending even into September, with 



VOL. II. 5 



