2(32 COMPARATIVE ANATOMY. 



antennae^ is a branch of the internal antennary nerve. Both struc- 

 tures thus represent the end organs of nerves, which are set in 

 vibration by the shaking- of the firm body (Otolith) which they 

 carry, and thus produce an excitation of the nerve. 



The general character of this remarkable system shows us how 

 the auditory organs arise from a differentiation of an indifferent 

 sensory organ connected with the integument. The auditory hairs 

 are only modifications of other " hairs ; ' of the integument which 

 contain nerve-endings, and just like those which may appear on the 

 free parts of the body (tactile rods). The formation of the unclosed 

 auditory vesicle, or auditory pit, represents therefore a second stage 

 in this differentiation ; and the change into a closed vesicle is a 

 further stage of this phenomenon. 



Hensen, Zeitpclir. f. wiss. Zool. XIII. 



§ 203. 



The other form of auditory organ is found in the Insecta. It is 

 principally in the Orthoptera, which are also provided with vocal 

 organs, that an organ for receiving the waves of sound can be made 

 out. The ordinary arrangement is a tympanic membrane, stretched 

 on a firm chitinous ring, one surface being directed to the exterior, 

 the other to the interior. On its inner side there is a tracheal vesicle, 

 and on this, or between it and the " tympanum," there is a 

 ganglionic nervous enlargement, from which specially modified 

 nerve-endings, having the form of small club-shaped rods, arise 

 between fine filaments. The tympanum, as well as the tracheal 

 vesicles, serves as an organ for conducting sound. The organs of 

 perception are represented by the nerve-endings, which are regularly 

 arranged. In the Acridida the organ lies in the nietathorax, just 

 above the base of the third pair of legs, and receives its nerve from 

 the third thoracic ganglion. The Locustida and the Achetida have 

 the organ embedded in the tibia of the two anterior legs. In the 

 former a tympanum lies on either side of this leg", either super- 

 ficially, or at the bottom of a cavity, which opens anteriorly by a 

 single orifice. Two tracheal branches occupy the space between the 

 two tympana, one of which carries the ridge-shaped nervous end- 

 organ. This auditory ridge is, in Locusta, formed by a series of cells 

 which grow smaller towards one end; each of these contains a 

 ' ' rod " of proportional size. The tympanum in the Achetida lies on 

 the outer side of the tibia of the anterior leg. 



Other organs, the nature of which is less definitely settled, are 

 allied to these, as by their general structure they represent auditory 

 oi'gans; the presence of the same pencil-shaped body in the termi- 

 nations of nerves justifies us in at least ranking these organs with 

 the auditory, while, further, such a relationship is implied by the 

 ganglionic outspreading of the proper nerve along a tracheal branch. 

 The ends of the nerves are directed towards the integument, the 



