EXTERNAL CHARACTERS. 87 



"thickened and pigmented, and bears 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 dor-sal 

 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 th€y 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 

 passages, which unite behind to form 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 in- 

 going or inhaled stream being ventral, the exhaled 

 dorsal. 



This current is maintained by the cilia of the 

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

 for nutrition and respiration, and to carry away the 



