XI.] THE FEESH- WATER MUSSEL. 117 



third. The pedal ganglia will thus be brought 

 into view. They are a pair of deep-orangc- 

 coloured oval bodies, each rather larger than a 

 big pin's head ; they are applied to one another 

 in the middle line. 



13. From each ganglion one commissural cord 

 (a. /3.) passes forwards and upwards to the 

 cerebral ganglion of its side, and branches are 

 given off to the muscles of the foot and to the 

 auditory organ. 



c. The parieto-splanchnic ganglia. 



a. This pair are readily found by turning the 

 animal on its dorsal side, and dissecting away 

 the integument from the ventral surface of the 

 posterior adductor muscle. 



/5. Trace forwards from each the cord (a. /3.) which 

 runs to the cerebral ganglion of the same side. 

 It is easy to follow the commissure so long as 

 it lies in the region of the organ of Bojanus 

 difficult further on. 



8. The auditory organ. 



a. This is rather difficult to dissect out in Anodon : 

 it is a small sac which may be found by tracing 

 back the posterior cord given off from the pedal 

 ganglion, to a branch of which it is attached. 

 There is usually an auditory vesicle connected 

 with each pedal ganglion. 



b. If a fresh Cyclas 1 be obtained, and its foot re- 

 moved, mounted in water, and examined with 



1 Cyclas cornea a small fresh-water lamellibranchiate mollusk. 



