XI. J THE FEESH-WATER MUSSEL. 117 



third. The pedal ganglia will thus be brought 

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

 coloured oval bodies, each rather larger than a 

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

 in the middle line. 



/3. From each ganglion one commissural cord 

 (a. ^.) 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. 



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

 runs to the cereb)ral 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. 



A 



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. 



6. If a fresh Cyclas ^ be obtained, an!d its foot re- 

 moved, mounted in water, and examined with 



1 Cyclai cornea — a small fresh-water lamellibranchiate mollusk. 



