xi.] THE FRESH-WATER MUSSEL. 111 



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. 



^ 7 b. If a fresh Cydas* be obtained, and its foot re- 

 moved, mounted in water, and examined with 

 i inch obj., the auditory sac can readily be seen 

 with a constantly-trembling particle, the otolith, 

 in it. 



9. The alimentary canal. 



a. This should be dissected out in another Anodon 

 which has been well hardened in spirit. Care- 

 fully dissect away the thin layers of muscle which 

 cover the left side of the foot: as this is done 

 the dark-looking coil of the intestine will come 

 into view: the two coils lying parallel to one 

 another near the posterior border of the foot 

 being probably those first seen. Continue to 

 pick away the muscles and reproductive caeca 

 until as much as possible of the course of the 

 intestine is exposed. Make a small hole in it in 

 one of the hindermost coils, pass in the end of 

 a blow-pipe and inflate : then carefully lay open 

 the intestine throughout its whole length so as to 

 expose its inner surface; working towards the 

 stomach on the one hand and the rectum on the 

 other. Pass a guarded bristle into the mouth as 

 far as it will readily go, and then lay open the 

 1 Cyclas cornea a small fresh- water lamellibranchiate mollusk. 



