2l6 HISTOLOGY 



that birds probably hear. Its value in the case of other vertebrates is 

 negative, while for all other creatures it is entirely valueless. The 

 presence of sound-making apparatuses in the animal is rather poor 

 evidence that an auditory power is also present. Many animals are 

 practically mute, and yet have the keenest of ears for the sounds made 

 by enemies. 



The voice of insects is often put forth as evidence that they must 

 hear. The plecypod mollusk, living on the bottom of the stream where 

 the current pouring over stones and sand must make a noise, might use 

 the otocyst to hear the noise. But the tiny Cyclas, living deep in the 

 mud and ooze of the stillest ponds, has really no possible use for its 

 otocyst other than to know which direction is up and which is down. 

 Most mollusks probably hear nothing. 



The presence of accessory tissues to gather and transmit the sound is 

 good evidence. The tympana and pinnae of various kinds tell unmis- 

 takably that an auditory function is at least a part of the organ's duties. 

 This is the determining factor in the frog, where experiment is uncertain. 

 Experiment is difficult, but some of its positive results are conclusive. 

 The only reaction that we can trust is a sudden motion or start when the 

 sound is made, and it is possible that many forms would not move even 

 if they heard the noise. A too loud sound might also stimulate other 

 sense organs. 



We shall study four forms of tissue that can undoubtedly perceive 

 sound : the auditory hairs of the mosquito and other insects ; the chor- 

 dotonal organs of an insect ; the ear of an insect, and the ear of a Guinea 

 pig, which much resembles that of man (for the possible auditory organ 

 of the cephalopod mollusks, see the part on equilibration). 



The hair-like auditory organs have been best studied in the antennae 

 of the male mosquito and in the auditory hairs of some larvae (Corethra). 

 The mosquito (a male) was fastened to a glass slide by the feet so that 

 he was living and in health, but quiet enough to be put under the micro- 

 scope and studied. Tuning forks were then sounded in a succession 

 of strong notes of various pitches, and it was observed that at certain 

 notes the hairs vibrated, strongest at 512 vibrations per second and 

 weaker at some adjacent notes and in some of the other hairs. The 

 vibrations of these hairs act upon a very peculiar and complex organ 

 found in the second basal segment of the antenna. A nerve carries the 

 stimulus from this organ to the brain. As this organ only acted when the 

 hairs were at the proper angle to the sound waves, and as this angle 

 extended from directly in front for some distance toward the outer side 

 of each antenna, it can be seen that the male mosquito can perceive 

 both the sound and its direction, and thus can find the female in the dark. 



The histology of this organ is, in principle, like that of the tactile 



