SCHARFF : ORGANS OF SENSE IN MOLLUSCA. 31I 



means of which they can distinguish simple musical sounds, has 

 as yet been discovered. 



The Olfactory Organ. 



The presence of this organ is as yet of a more or less 

 problematical nature, and I therefore purpose dealing with it 

 very briefly. 



With regard to the Lamellibranchs, it appears to be pretty 

 well established that a smelling organ exists. A structure richly 

 supplied with nerves has recently been pointed out by Spengel in 

 several marine forms. Subsequently he also discovered the same 

 in Anodon and other freshwater shells. Its position is just where 

 we should expect to find it, viz. : at the hinder base of the gills. 

 Here the water entering the shell has to pass, and can be 

 scrutinized by the smelling organ as to its salubrious nature. It 

 consists of a specially modified crest of epithelium, and lies 

 close to the visceral ganglion, with which the so-called olfactory 

 ganglion, underneath the organ of smell, is connected. 



A structure occupying a similar position, viz., at the 

 entrance to the lungs in pulmonates, was first demonstrated by 

 Lacaze-Duthiers, and it is now known among Zoologists as ' the 

 organ of Lacaze.' Neither its discoverer, nor Simroth, a later 

 writer, appear to have anticipated an olfactory organ in this 

 structure. In fact, as it does not exist in terrestrial pulmonates, 

 at any rate not in Helix, Simroth draws rather a far-fetched 

 conclusion. He believes that it represents a sense, whose 

 function it is to acquaint the aquatic pulmonates, when they 

 have arrived at the surface of the water, in order to breathe. 



In the genus Planorbis it is on the left side, in Limngea and 

 Physa on the right. Spengel declares that there is no doubt of 

 the olfactory nature of the organ of Lacaze, and Fol, in his treatise 

 of the ' Development of MoUusca,' expresses a similar opinion. 



Lastly, I have to mention an organ in the lung-chamber of 

 Parmacella oltvieri, which was described a short time ago by 

 Simroth, and which is supposed to be a smelling organ. I am 



