88 THE FEESHAVATER MUSSEL. 



fffical matter and excretory and reproductive pro- 

 ducts. 



a. The inhalant aperture, through which the inhaled 



stream enters the branchial chamber, is a vertical 

 slit at the hinder end of the body : it is bordered 

 laterally by the thickened posterior edges of the 

 mantle-lobes, which bear the tentacular fringes. 

 Below, it is incompletely closed by the approxi- 

 - mation of the edges of the mantle-folds ; and 

 above, it is separated from the exhalant aperture 

 by a horizontal partition formed by the fusion of 

 the gills of the two sides with each other. 



b. The exhalant or cloacal aperture is much smaller 



than the inhalant opening, and lies immediately 

 dorsal to this, and behind the posterior adductor 

 muscle. It is bounded laterally by the thickened 

 mantle-borders, which have here no tentacles. 



Pass a seeker into the cloacal opening, and forwards along 

 the supra-hranchial cavity above the gills. Note thepartition 

 between the exhalant and inhalant apertiires. 



6. Organs lying in the branchial chamber. 



Turn bach the right mantle-lobe as fully as possible. 



a. The foot and visceral mass form a large laterally 



compressed oblong mass, about half the length 

 of the shell, and lying between the two adductors. 

 The upper two-thirds, which are paler in colour, 

 form the visceral mass : the lower or ventral third 

 forms a powerful muscular foot, orange-red in 

 colour, which can be protruded between the valves 

 in front, and is used by the animal for working 

 its way along the bed of the pond in which it 

 lives. 



b. The gills are two pairs of large lamellar organs 



lying at the sides of the visceral mass, between 

 it and the mantle, and extending back beneath 



