xi.] THE FRESH-WATER MUSSEL. 119 



ventral edges and enclosing a central cavity, 

 which opens into a chamber (epibranchial} above, 

 which is continued back to open into the cloacal 

 chamber. The cavity between the lamellae is 

 subdivided by irregular partitions, which pass 

 from one lamella to the other. 



b. Carefully cut out a bit of the wall of the gill-sac 

 on one side; mount in water and examine with 

 i inch obj. The outer surface will be seen to be 

 formed by parallel vertical bars, containing pairs 

 of short rods; the inner face being formed by a 

 meshwork of large vessels, perforated by wide 

 apertures. 



c. Examine with a higher power: the margins of 

 each cleft will be found covered with large active 

 cilia. 



7. The nervous system. 



a. The cerebral ganglia. 



a. These will be found by carefully dissecting 

 away the bases of the labial palps and the 

 integument on the dorsal side of the mouth. 

 They are two in number, each about the size 

 of a pin's head, and somewhat triangular in 

 form. 



/3. The commissures connected with the cerebral 

 ganglia are 



A short cord uniting the two ganglia across 

 the middle line over the mouth. 



A cord, the cerebro-pedal commissure, which 

 runs downwards and backwards from each and 

 becomes continuous with that which runs for- 



