The Senses of Animals. 265 



stone tap against an opaque glass plate standing vertically 

 in the water." Carp at Potsdam are, indeed, said to come 

 to be fed at the sound of a bell. But Mr. Bateson well 

 remarks that this " can scarcely be taken to prove that the 

 sound of the bell was heard by them, unless it be clearly 

 proven that the person about to feed them was hidden from 

 their sight." There is clearly room for further observation 

 and experiment in this matter. 



Turning to the invertebrata, we find, even in creatures 

 as low down in the scale of life as jelly-fish, around the 

 margin of the umbrella in certain medusae, simple auditory 

 organs. In some cases they are pits containing otoliths 

 (minute calcareous or other bodies, which are supposed to 

 be set a-danee by the sound-vibrations) ; in others there is 

 a closed sac with one or more otoliths ; in others, again, 

 they are modified tentacles, partially or completely enclosed 

 in a hood. All these are generally regarded as auditory, 

 there being specially modified cells of the nature of hair- 

 cells. We shall see, however, that another interpretation 

 of organs containing otoliths is at any rate possible. For 

 the present, we will follow the usual interpretation, and 

 regard them as auditory. 



Vesicular organs containing otoliths are found near the 

 cerebral ganglia in some of the worms and their relations. 

 But the common earthworm, though it appears to be sensi- 

 tive to sound, does not appear to have any such organs. 



Molluscan shell-fish are generally provided with auditory 

 organs. In the fresh-water mussel it is found in the 

 muscular foot. It can be more readily seen in the Cyclas, 

 if the transparent foot of this small mollusc be examined 

 under the microscope. It is a small sac containing an 

 otolith. Mr. Bateson found that the mollusc Anomia "can 

 be made to shut its shell by smearing the finger on the 

 glass of the tank so as to make a creaking sound. The 

 animals shut themelves thus when the object on which they 

 were fixed was hung in the water by a thread." In the 

 snail and its allies the auditory sac is found in close 

 connection with the nerve-collar that surrounds the gullet. 



