PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA.. 221 



motious, but when, in addition to this, both otocysts were destroyed, 

 compensatory movements completely disappeared. 



Bethe ('97), in his physiological work on Carcinas msenas, confirms 

 Clark's results. In a previous paper he ('95*) observes that Mysls can 

 hear after the otocysts have been destroyed, but with difficulty ; also that 

 the animals are more sensitive to low tones than to high. 



Thus, until 1898 three views were held as to the function of the 

 otocysts : 



(1) That they are purely auditory organs (Hensen and the earlier 

 zoologists). 



(2) That they are both auditory and static in function (Delage and 

 Bethe). 



(3) That they are purely static in function, i. e. organs of orientation 

 (Kreidl, Clark, and others). 



To determine whether decapod Crustacea really hear, and if so, 

 whether the otocyst is the organ of audition, is the aim of two papers 

 by Beer ('98, '99). 



In criticising the conclusions reached by Hensen and Bethe, Beer 

 remarks in his first paper that, because decapods were made to react to 

 different sounds, does not prove that these Crustacea responded to true 

 sound, or that they heard. These reactions may have been due to their 

 feeling vibrations transmitted to the water from the walls of the vessel 

 in which they were confined, — a tactile reaction, or, to use Bethe's term, 

 a "tango-reflex." Experiments with sounds produced in the air Beer 

 considered superfluous, as it is a well-known physical fact that most of 

 the sound waves are reflected from the surface of water. 



Beer found that Crustacea reacted strongly to sounds produced in the 

 water by striking partially submerged bells, jars, etc., but only when 

 tliey were not at a greater distance from the source of sound than that 

 at which vibrations could be detected by the hand immersed in water. 

 The animals responded more strongly when near the walls of the vessel ; 

 but vibrations could be felt by the band also in this position more dis- 

 tinctly, even though further removed from the source of the sound. 



For animals well supplied with tactile organs, he regards pure sound 

 or pure audition as impossible ; because vibrations could be felt as soon 

 as heard, and, this being the case, audition would be useless. 



On removal of the otocysts, Palsemon and Palaemonetes still responded 

 to sound waves produced in the water. There was, however, a slight in- 

 hibition of the customary reactions, therefore the hairs of the otocyst 

 are probably slightly tactile as well as static in function. 



