226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(3) Removal of the antennz and antennules which are supplied with 
numerous tactile bristles, inhibits the reaction. 
(4) Decapods, such as Virbius, normally without otocysts respond 
vigorously ; but removal of antennz 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 reactions 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. This 
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 Gelasimus pugilator (Brachyuran 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 the Ma- 
erura. They always rested upon the 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 
