V.] THE FRESH- WATER MUSSEL. 33! 



upwards and forwards, from the pedal to the cere- 

 bral ganglion of either side. 

 (Cf. Sect. L./) 



The cerebro-splanchnic commissures run close 

 together and parallel with each other, in their 

 course from behind forwards ; on leaving the peri- 

 cardialarea, they suddenly diverge and pass down- 

 wards and forwards to meet the cerebral ganglia. 

 If one of the commissures be removed and 

 examined under a low power, there may occa- 

 sionally be found, at the point of divergence, an 

 enlargement (visceral ganglion}. 



J. The auditory organ. 



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

 it is a small sac which may be found by tracing 

 back the posterior cords given off from the pedal 

 ganglion, to a branch of one of which it is attached. 

 There is a vesicle connected with each pedal 

 ganglion. 



Failing the above, the otocyst can best be found 

 by removing a portion of the reproductive gland 

 lying immediately behind the pedal ganglion, and 

 examining under a low power. 



b. If a fresh Cyclas 1 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. 



K. The shell or exoskeleton. 



a. Its two hardened lateral pieces or valves ; each 

 with a straight dorsal and a curved ventral edge, 

 1 Cydas cornea a small fresh- water lamellibranchiate mollusk. 



