226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



(3) Eemoval of the antennreand autennviles which are supplied with 

 miraerous tactile bristles, inhibits the reaction. 



(4) Decapods, such as Virbius, normally without otocysts respond 

 vigorously ; but removal of antennae and antennules diminishes their 

 sensibility in a marked degree. 



(5) Precisely the same responses as were called forth by the produc- 

 tion of sound were also obtained by simply tapping or jarring the walls 

 of the aquarium. 



Whether due to tactile stimulus or to audition, the fact remains, that 

 the otocyst has little or no part in producing the i-eactions observed in 

 the series of experiments ; for (1) decapods normally without otocysts 

 respond as vigorously to the same stimuli as those possessing them, and 

 (2) the removal of the sacs from the latter has only a very slight in- 

 hibiting effect, which might be due either to the loss of these organs, 

 or to the injury of the nerves supplying the many tactile bristles of the 

 antennule. 



Consequently, the otocyst not being the organ by stimulation of which 

 responses to sound vibrations are called forth, and there being no other 

 sensory apparatus in Crustacea especially differentiated for the reception 

 of sound waves, we are led to the conclusion that in decapod Crustacea 

 a true auditory organ is wanting. 



The acute tactile sense of decapods may to some extent serve the 

 same purpose that audition does in vertebrates. In mammals the senses 

 of touch and hearing grade into each other. The range of the average 

 auditory organ in mammals is from 30 to 16,000 vibrations per second ; 

 waves of less than 30 vibrations per second do not usually produce audi- 

 tory sensations, but are appreciable to the tactile sense. It is important 

 to note that decapods respond most vigorously to low notes, and not at 

 all to high notes or sounds produced by very rapid vibrations.' Tiiis 

 fact would seem to be good evidence that the vibrations imparted to 

 the water and perceived by decapods correspond to those which produce 

 tactile rather than auditory sensations in vertebrates. 



2. Responses of Gelasimns pttgihttor {Brachi/uran decapod). 



a. To Vibrations transmitted to Water. On the conduction of sound 

 waves to water by the same means as in the preceding experiments, these 

 fiddler crabs responded, but by no means as vigorously as did tlie Ma- 

 crura. They always rested upon tlie bottom of the aquarium, and 

 reacted by retiring slowly, either from the source of the sound, or from 

 the vibrating walls of the aquarium. In either case the response took 



