Eyes of MoUuscs und Aitliiopods. 721 



but little more coiild bc said of any auditoiy orgau; yet, iu spite of this 

 fact the Medusa would bc infinitely removed from hearing. But such 

 a structure could at once become a true auditory organ as 

 soon as the animai was sufficiently organized to distin- 

 guish. by association. certain vibrations from others, so 

 that some would cause contractious, and others, equally 

 strong. would not. This definition is an arbitrary one, but it ap- 

 pears probable that some such changc must havc taken place in all audi- 

 tory Organs, at one time or auother, just as in the eye. It would be, 

 however, extremely difiicult to decide, in innumerable cases, whether a 

 certain animai with an otolithic sac simply received a Constant current 

 of Stimuli through it, or whether it was able to associate certain kinds 

 of Vibration with certain conditions, so that different muscular contrac- 

 tious would follow. This, however, earries us away from our snbject 

 into the uncertain domain of the origin of the will. 



Physiologists consider that the Constant semi-contracted eondi- 

 tion of the muscles in the human body, is due to a Constant stream of 

 impressions, however minute, received from the skin and sense organs. 

 It is a well known fact that the otoliths of a great many Mollusca 

 and Worms are in a Constant state of Vibration, so that a stream 

 of energy must be transmitted, by the sense hairs receiving the blows 

 of the otoliths , through the nerves , to the various centres where the 

 consumptiou of energy takes place. 



Another form of sense hair cells is that which records the impres- 

 sions produced by various changes in the chemical composition of tluids 

 or gases, giving rise to the perception oftaste or smeli. These pro- 

 cesses are much more obscure than those of hearing. Smeli and taste 

 cells are. undoubtedly, modifications of the sanie Clements, for function- 

 iug under different conditions ; the distinction between them is without 

 doubt. wanting iu the Invertebrates, and possibly in many Vertebrates. 

 It is possible that the so-called olfactory pits of the Coelen- 

 terates, Mollusca , and Worms arose as modifications of ciliated respi- 

 ratory cells. Let us suppose a patch of ciliated cells, either used for 

 locomotion or respiration ; now it is known that in some way or other 

 carbonic acid gas acts as a Stimulus, and causes, for a certain time, 

 renewed activity; it is not improbable that other noxious substances 

 would produce the same effect. It is also probable that some of the 

 cells would be more aft'ected by certain Compounds than others. Such 

 cells. from being at first simply stimulated by certain fiuids or Com- 

 pounds, would. by a series of changes exactly similar to those under- 



Mittheilnngen a. d. Zoolog. Station zu Neapel. Bd. VI. 48 



