82 THE FRESHWATER MUSSEL 



At the hinder end of the animal, below the 

 posterior adductor, the margin of the mantle which 

 here bounds the inhalent a,perture, is very greatly 

 thickened and pigmented, and bears a fringe of ten- 

 tacular 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. The pallial openings. If a mussel be placed in a vessel 



of water with a layer of mud or sand at the bottom, 

 and left to itself, it will assume the position described 

 above, the anterior end being buried somewhat 

 obliquely in the mud, and the posterior end project- 

 ing into the water. The valves will be slightly 

 opened, and streams of water, which can be rendered 

 clearly visible by adding a little colouring matter, 

 will pass in and out of the posterior end, the ingoing 

 or inhalent stream being ventral, the exhalent 

 dorsal. 



This current is maintaiiied by the cilia of the 



