MOLLUSCA. 9 



tentacles. In the apathetic Elysiidce and Bullidw the 

 eyes are sessile and rudimentary ; but in the active 

 and carnivorous Strombidw, and in the air-breathing 

 HelicidtB, they are elevated on peduncles, to allow a 

 wider and freer range of vision, 



Mollusca in general are dumb ; the only instances to 

 the contrary are the Cephalopods, which squeak and grunt 

 when removed from the water, and the Tritonia and Eolis 

 punctata, which produce a sound like the clink of a steel 

 wire on the side of a jar. The noise produced by snails 

 crawling on window-panes is merely mechanical. The 

 Tritonia arhorescens emits audible sounds under water, 

 which, no doubt, are meant to be heard by other indi- 

 viduals, but the instrument adapted to receive sonorous 

 undulations has not been detected ; these lower forms may 

 be said to hear with the whole surface of their body. 

 Where it has been observed, the organ of hearing in 

 Gasteropodous Mollusks is in the form of two round 

 vesicles containing fluid and certain oval, calcareous, or 

 crystalline oscillatory bodies situated on the head. In 

 the bivalves there is only one otolith of large dimensions, 

 which fills the cavity of the vesicles. The internal ear 

 is most developed in the Cephalopods, some of which have 

 even an indication of an external ear. 



Smell exists in the Mollusca to a certain degree, but 

 its organ has not yet been detected. Cuvier thought it 

 resided in the soft mucous skin ; the labial appendages, 

 the respiratory apertures, and the surface of the tentacles 

 may all receive impressions of odoriferous bodies. M. 

 Valenciennes regards the hollow plicated process beneath 

 the eye in the nautilus as an organ of smell. Taste may 

 be exercised by the sensitive buccal appendages and soft 

 membranes of the pharynx, but the tongue is a mere 



VOL. I. 



