EXTERNAL CHARACTERS 81 



posterior adductor, the margin of the mantle, which 

 here bounds the inhalent aperture, is very greatly 

 thickened and pigmented, and produced into a fringe 

 of tentacular processes. 



Immediately behind the posterior adductor the 

 mantle-lobe fuses with its fellow of the opposite side, 

 and the fused borders run forwards along the dorsal 

 surface of the posterior adductor for about half an 

 inch. They then separate and continue forwards as 

 a pair of narrow fringes, bounding a shallow groove, 

 for about half an inch further, where they cease. 



3. The left mantle-lobe, which can as yet be only imper- 



fectly seen, agrees exactly with the right, the animal 

 being bilaterally symmetrical. 



4. The pallial or mantle-cavity is the space between the 



right and left mantle-lobes. In it lie the foot and 

 the greater part of the visceral mass, the gills, and 

 other organs. 



It is divided by a horizontal partition, formed by 

 the bases of the gills, into two chambers of very 

 unequal size ; (1) the large ventral branchial 

 chamber; (2) the smaller dorsal supra-branchial 

 chamber, the hinder end of which forms the cloacal 

 chamber. 



5. 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 



