1910.] 



Tone Perception in Gammarus pulex. 



393- 



although there can be no doubt as to the genuine character even of the first, it 

 is only to the second that I have considered it safe to attach any importance. 

 In the one case the first antenna3 are observed to give a distinct neurotic 

 flicker, but in the other they are rapidly and powerfully flexed, being bent 

 right underneath the body, and carrying the passive second pair of antennae 

 before them. In both cases it is important to note that we have to do- 

 with a definite physiological response, which unquestionably supervenes as 

 the result of the stimulus, and is not a merely physical effect such as Hensen 

 observed in the auditory sette of the Decapod Crustacea, which effect has 

 since been shown to have no necessary auditory significance. In my earlier 

 experiments no record was, unfortunately, made of the sex of the specimens 

 employed. This was, however, done in the later work, but no difference 

 between the sexes was observed. In a very few cases the response was of a 

 character different from either of those just described, the anterior antennae 

 being only partially flexed at the first stimulus, and, instead of returning to- 

 the status quo, becoming more and more flexed with each successive stimulus,, 

 until they finally reached a position similar to that attained by the one 

 flexure in the case of the second type of response above. Tested dry out of 

 water directly on the resonator, and on the organ pipe, some individuals- 

 showed very distinct signs of disturbance to notes immediately above and. 

 below middle C, but they did not hop. 



In Gammarus the auditory function is generally supposed to be located ini 

 the first antennae, and, that this is so, is proved by the following experiments.. 

 Four pairs in copula were separated, and two of each sex had either the first' 

 or the second pair of antennae removed. This is easily done by holding the 

 animal down with a camel's hair brush and detaching the antennae with a 

 sharp needle or a very fine pair of forceps. The blood at once clots over the 

 wound, leaving a characteristic black scar. Both males and females on the 

 resonator responded to middle B flat after removal of the second antennae, in 

 some cases only to the B flat, and in one case markedly to the B flat, and less- 

 so to a few semitones below it. On no occasion, however, was the slightest 

 response observed after the first antennae had been removed. A female 

 minus the second antennae was tested on the physical table, and responded 

 distinctly to B flat when the bell of the trombone was not more than 6 inches- 

 from the microscope, but failed to respond when the bell was withdrawn to 

 18 inches. All the above individuals were tested again 10 days after the 

 operation (except one female which had died), and whilst there was still no 

 response whatever from those without the first antennae, those with these 

 appendages only responded in two cases, and then but slightly. 



Fatigue appears to play an important part in the results of the experiments. 



