276 ORGANS OF THE SENSES. 



both in the molluscous and articulated tribes, and they 

 relate to movements of the surrounding element which give 

 notice of objects at a distance. The percussions given 

 by outward bodies to the air or water in which animals 

 reside, are communicated, like undulations of light, to the 

 general surface of their body, and may produce some feeble 

 sensation where there are yet no distinct acoustic instru- 

 ments developed, as light appears to affect many organized 

 beings without eyes. But most of the higher invertebrated 

 animals and all the vertebrata present at the distal expanded 

 soft extremities of distinct auditory nerves, more or less 

 complicated acoustic instruments adapted to receive and 

 transmit sonorous undulations, and to render more distinct 

 the perception of their force, their direction and their ra- 

 pidity of succession. These, like most of the other organs 

 of sense, are disposed symmetrically on the two sides of 

 the head,, and the impression of sound may be communicated 

 to the auditory nerves without passing through the external 

 acoustic apparatus, as by means of the solid parietes of the 

 head, or by direct impression on the nerves from within. 

 From the structure of the organ of hearing it is more adapted 

 for communicating aereal vibrations than those of water, and 

 it is most general and most developed in the air-breathing 

 classes. The sudden percussions communicated by various 

 means to the dense watery element in which most of the 

 lower invertebrata reside, may affect rapidly and powerfully 

 the whole surface of their body, whether naked or covered 

 with hard vibratile parts, and through that means the most 

 sensitive internal parts, without these animals having special 

 acoustic organs to concentrate the undulations and direct 

 them to particular nerves, and we ascend as high as the 

 active-air-breathing insects before we perceive distinct organs 

 appropriated to this sense. Many insects have hard organs 

 for producing audible sounds by their rapid attrition against 

 each other, and these sounds are often heard and repeated 

 by their mates, being a means of communication between the 

 sexes, especially in the darkness of the night. The organ 

 of hearing in insects already presents not only a distinct 

 auditory nerve and vestibule, the first and most essential 

 elements of all this acoustic apparatus, but also the rudi- 

 ments of two semicircular canals, according to the obser- 



