228 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



of sound production among decapods, observed by T. Parker ('78) and 

 Goode ('78), are (1) the stridulatiou of the rock lobster, Palinurus, where 

 the sound is produced by rubbing the second segment of the antenna 

 against the antennule, and (2) the pistol-like report produced by Alpheus 

 in snapping together the claws of the great chela. As Beer has pointed 

 out, the otocyst is poorly developed in Palinurus ; furthermore, no in- 

 dividuals of either species have ever been observed to respond in any way 

 when these snappiugs or stridulations wei'e produced. 



We can no more argue, from these two instances of sound production 

 in decapods, that there is an auditory function in all Crustacea than 

 we can that all fish hear because the drum-fish makes a sound. 



The enemies of water-inhabiting crustaceans produce no sound3 

 which would reveal their presence to their prey; the latter would 

 therefore liave to rely upon other forms of stimulation for the detec- 

 tion of their foes. Even if it were admitted that they possessed a sense 

 of hearing, yet, as shown both by Beer's experiments and by my own, 

 it must be so restricted in range that they would be able to detect 

 sound produced at no greater distance than that at which the vibra- 

 tions could be felt by the hand. Such a dull sense as this would be 

 of no practical value in protecting crustaceans from their foes. 



Both observation and experiment lead, then, to the following general 

 conclusions : 



(1) The reactions formerly attributed to sound stimuli are nothing 

 more than tactile reflexes. 



(2) The otocyst has little or no part in calling forth these reactions. 



(3) There is no direct evidence to prove that decapod Crustacea 

 hear, and until such evidence has been obtained, we are not warranted 

 in ascribing to the otocyst a true auditory function. 



II. The Otocyst as an Organ of Equilibration. 



All water-inhabiting, free-swimming animals which maintain a defi- 

 nite position with reference to gravity either during locomotion or 

 when at rest, can thus orient themselves only under one or the other 

 of two conditions : 



Either the animal must be normally in a condition of stable equi- 

 librium, keeping its definite position under the influence of gravity Hke 

 any inanimate body; or, if a position of unstable equilibrium is main- 

 tained, the animal must in some way be made sensible of the direction 

 of gravity, and must keep itself in equilibrium by its own efforts. 



