3o8 SCHARFF : ORGANS OF SENSE IN MOLLUSCA, 



Having concluded my observations on the eye, I shall now- 

 pass to the organ of hearing in the various groups of land and 

 freshwater shells. 



The Organ of Hearing. 



An auditory organ or otocyst appears to be more universally 

 present among our native shells than the organ of sight. The 

 only one of our Lamellibranchs in which it has not been found 

 is Dreissena polymorpha, but it is probable that it possesses an 

 otocyst during youth, which becomes lost later on. 



In order to demonstrate the ear, we may take a large shell 

 like Anodon, and after having removed its valves and gills, slice 

 away the yellowish connective tissue of the foot by means of 

 horizontal incisions. Great care should be taken in this process 

 for fear of damaging the pedal ganglia. Many specimens will 

 have to be dissected before a satisfactory result is arrived at, 

 when two small knots will be seen apparently in close connection 

 with the ganglia. These are the ears. On account of their 

 position near the pedal ganglia, the otocysts were commonly 

 believed to be supplied with nerve force from them. 



Lacaze-Duthiers was the first to point out the relation of 

 the octocysts to the cerebral ganglia in Gasteropods (see ' Arch- 

 ives de Zoologie Experimentale,' 1872). It is only compara- 

 tively recently that a similar condition was demonstrated in 

 bivalves. This discovery is due to Simroth. The auditory 

 nerve runs along with the commissure from the cerebral to the 

 pedal ganglia ; before reaching the latter, however, it branches 

 off to the ears (see fig. 3). 



In the family of the Unionidse the otocysts are surrounded 

 by a spongy capsule^ the meshes of which are filled with some 

 fluid, presumably water and blood. This peculiar structure 

 will be referred to again in the consideration of the physiologi- 

 cal function of the ears. 



For an examination of the more minute details of the organ 

 of hearing, the freshwater shell Cyclas cornea is a very suitable 



J.C., iv., April, 1885. 



