THE MANTLE-CAVITY 83 



mantle, gills, and palps : it serves to bring water 

 for nutrition and respiration, and to carry away the 

 faecal matter and the excretory and reproductive 

 products. 



a. The inhalant aperture, through which the inhalent 



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 exhalent aperture 

 by a horizontal partition formed by the fusion of 

 the gills of the two sides with each other. 



b. The exhalent or cloacal aperture is much smaller 



than the inhalent opening, and lies immediately 

 dorsal to it, 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-branchial cavity above the gills. Note the partition 

 between the exhalent and inhalent apertures. 



6. Organs lying in the branchial chamber. 



Turn back 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, reddish in colour, 

 which can be protruded from between the valves 

 in front, and is used by the animal for working its 

 way along the bed of the stream 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 

 the posterior adductor to the hinder end of the 



G 2 



