462 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



insect, and the bow on which it is stretched is merely 

 a part of the same integument that has become con- 

 siderably thickened. Taking for an example the 

 grasshopper, we find a number of small muscles 

 which are inserted into the bow, and by their exten- 

 sion or contraction increase or diminish the tension 

 of the tympanic membrane. The central portion of 

 the membrane is occupied by a cavity which commu- 

 nicates with the exterior by an open tube, and within 

 the cavity there is a ganglionic mass developed at the 

 end of an auditory nerve. Here, then, we have the 

 case of the sonorous vibrations impinging on a mem- 

 brane, which is held tense ; being conveyed to an air 

 chamber, which is in relation with the outer air, and 

 is therefore capable of adapting itself to any force that 

 may be brought to bear upon it ; and being carried 

 thence to the termination of a sensory nerve. 



In some insects, though not in the Orthoptera, 

 rows of corpuscles have been observed on some of the 

 nervures of the wing, and as these are supplied witli 

 nerve filaments, Braxton Hicks has suggested that 

 they are auditory organs ; somewhat similar organs 

 found on the halteres of the Diptera have had the same 

 function ascribed to them by Lowne. 



In the ears, as in some other parts of the organi- 

 sation of the Mollusca, we see arrangements which 

 are simpler, and others that are more complex than 

 those that obtain in the Arthropoda. The auditory 

 organ is often a simple closed vesicle, surrounded by an 

 investing membrane, and having in its cavity sensory 

 cells provided with projecting hairs (Fig. 194); the 

 central cavity is occupied by a single large concretion, 

 otolitli, or a number of smaller otoconia, as the 

 smaller concretions may conveniently be called. In 

 the Nautilus, the ears, as in most Lamellibranchs and 

 Gastropods, are attached to the pedal ganglia, but in 

 the Dibranchiata they are enclosed in the cartilage 



