PRENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 225 



The above experiments were duplicated on Crangon vulgaris with 

 similar, though less marked results, as Crangon is much more sluggish 

 than Palaemonetes. 



A third set of experiments was tried with Virbius zostericola, a 

 shrimp-like decapod without otoeysts. Normal animals responded vigor- 

 ously on striking a glass jar partially submerged beneath the water in 

 which they swam. This response, much increased by strychnine 

 poisoning, was distinctly diminished when both antennae and antennules 

 were removed. 



e. Meaning of these Experiments. All of my experiments confirmed 

 the conclusion of Beer, that free-swimming decapods, whether possessing 

 otoeysts or not, will respond to stimuli which are transmitted to them 

 by the liquid medium they inhabit. The next question is, to determine 

 whether this response is caused by the perception of sound waves or by 

 the coarser vibrations or jars imparted to the water. In other words, 

 have we to do with true audition or with the sense of touch'? 



Beer has clearly shown that there is no such thing as the transmission 

 of pure sound waves from air to water. Coarser waves are imparted to 

 the liquid simultaneously with those of sound, and can readily be felt 

 by the immersed hand. 



After making a number of trials with sounds produced as in the pre- 

 ceding experiments, I ascertained that the vibrations not only could be 

 plainly felt by the submerged hand, but also that they could be felt at 

 a distance from 10 to 20 cm. greater than that at which the shrimps ivould 

 react. This fact does not at all prove that the animals experimented 

 with do not hear, but merely shows that the responses supposedly pro- 

 duced by sound stimuli may be simple tactile reflexes, called forth by 

 vibrations which, since appreciable to the immersed fingers, we may cer- 

 tainly assume to he felt by these animals, so well supplied with delicate 

 tactile organs. 



That the reaction is really due to tactile stimulus rather than to audi- 

 tion, is indicated by several facts brought out by the experiments : 



(1) Animals, when near the wall of the vessel, even though distant 

 from the source of the sound, respond vigorously, leaping r>way from the 

 wall and toward the sound. The wall is set into vibration by the pro- 

 duction of the sound, and it is apparently this vibration which affects 

 them, rather than the true sound-waves imparted to the water. 



(2) The average distance from the source of the sound at which they 

 will respond is less than that at which vibrations may be felt by the 

 hand. 



