86 THE FRESHWATER MUSSEL 



iii. The posterior part, behind the foot, is fused 

 with the corresponding lamella of the gill of 

 the opposite side, forming the partition between 

 the inhalent and exhalent chambers of the 

 mantle-cavity. 



3. The supra-branchial or exhalent passages are wide 

 longitudinal channels running just above the bases 

 of the gills, and continuous with the interlamellar 

 spaces of the gills. 



Alongside the foot there are two such passages 

 on each side, one above the outer gill, and one above 

 the inner gill. The latter communicates with the 

 large branchial chamber through the longitudinal 

 slit between the inner lamella of the gill and the 

 hinder part of the visceral mass. 



Behind the foot there are only three passages, 

 the two inner ones becoming continuous with each 

 other ; and beneath the posterior adductor all three 

 unite to form a wide shallow passage, opening behind 

 into the cloacal chamber. 



Pass a seeker into the cloacal cavity, and then forwards 

 along the supra-branchial passages above the bases of the 

 gills. Lay open first the outer passage of the right side, 

 and then the inner one ; taking care not to damage the kidney, 

 which lies immediately above them. Note that the inner pas- 

 sages of the two sides are continuous with each other behind 

 the foot, and that they communicate with the branchial cavity 

 through the slits between the inner gills and the body. 



The respiratory stream of water, kept up by the 

 cilia on the mantle and gills, enters at the inhalent 

 aperture, passes forwards along the branchial cavity, 

 and then makes its way through the meshes of the 

 gills into the supra-branchial passages. It then passes 

 back along the supra-branchial passages to the cloacal 

 cavity, and so out. 



Respiration is probably effected mainly by the 

 inner surfaces of the mantle-lobes, and not by the 

 gills. 



