82 THE FEESHWATEE MUSSEL 



it and the mantle, and extending back beneath 

 the posterior adductor to the hinder end of the 

 mantle-cavity. The outer gill is, in the female, 

 often greatly distended with embryos. 

 c. The labial palps are two pairs of triangular folds 

 lying at the sides of the anterior part of the vis- 

 ceral mass, between the anterior adductor and the 

 gills. Each pair consists of a couple of lamellae, 

 of about equal size, attached along their dorsal 

 borders to the body-wall, and marked on their 

 apposed surfaces by close-set transverse ridges. 

 6. The external apertures. 



a. The mouth is in front, between the anterior adductor 



and the anterior border of the foot. 



Pass a seeker into the mouth, and note that it passes verti- 

 cally upwards behind the anterior adductor, into the stomach. 



The mouth is bordered by anterior and pos- 

 terior lips, formed by the labial palps. The outer 

 palp is continuous with its fellow of the opposite 

 side, forming a low ridge-like anterior lip, imme- 

 diately behind the anterior adductor. In a similar 

 manner the two inner palps are continuous with 

 each other and form a posterior lip, in front of 

 the foot. 



The deep groove between the outer and inner 

 palps of each side thus leads directly into the 

 mouth. The palps are richly ciliated, so that 

 any food-particles that come within the range of 

 their action are swept into the groove and carried 

 forwards to the mouth. 



b, The pallial openings. If a mussel be placed in a 



vessel of water with a layer of mud or sand at the 

 bottom, and left to itself, it will assume the 

 position described above, the anterior end being 

 buried somewhat obliquely in the mud, and the 

 posterior end projecting into the water. The 

 valves will be slightly opened, and streams of 

 water, which can be rendered clearly visible by 



