100 THE FRESHWATER MUSSEL. 



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i. The anterior part of the dorsal border, about a 

 quarter of its total length, is attached to the 

 side of the visceral mass. 

 ii. The middle portion has a free edge, and extends 

 back as far as the hinder border of the foot. 



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 inhalant and exhalant chambers of the 

 mantle-cavity. 



3. The supra-branchial or exhalant 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 the 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 passages 



of the two sides are continuous with each other behind the foot, 



and lliat 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 inhalant 

 aperture, passes forwards along the branchial cavity, 



