AUDITORY ORGANS. 513 



undulatory movements in the water, caused by the swimming of 

 the latter. 



Auditory organs with the above characters, sometimes freely 

 open to the external medium, but more often closed, are found 

 in various Ccelenterata, Vermes and Crustacea, and universally 

 or all but universally in the Mollusca and Vertebrata. 



In many terrestrial Insects a different type of auditory organ 

 has been met with, consisting of a portion of the integument 

 modified to form a tympanum or drum, and supported at its 

 edge by a chitinous ring. The vibrations set up in the mem- 

 branous tympanum stimulate terminal nerve organs at the ends 

 of chitinous processes, placed in a cavity bounded externally by 

 the tympanic membrane. 



The tympanum of Amphibia and Amniota is an accessory 

 organ added, in terrestrial Vertebrata, to an organ of hearing 

 primitively adapted to an aquatic mode of life ; and it is interest- 

 ing to notice the presence of a more or less similar membrane 

 in the two great groups of terrestrial forms, i.e. terrestrial Verte- 

 brata and Insecta. 



Nothing is known with reference to the mode of develop- 

 ment or evolution of the tympanic type of auditory organ found 

 in Insects, and, except in the case of Vertebrates, but little is 

 known with reference to the development of what may be called 

 the vesicular type of auditory organ found in aquatic forms. 

 Some very interesting facts with reference to the evolution of 

 such organs have however been brought to light by the brothers 

 Hertwig in their investigations on the Ccelenterata; and I 

 propose to commence my account of the development of the 

 auditory organs in the animal kingdom by a short statement of 

 the results of their researches. 



Ccelenterata. Three distinct types of auditory organ have 

 been recognised in the Medusae ; two of them resulting from 

 the differentiation of a tentacle-like organ, and one from ecto- 

 derm cells on the under surface of the velum. We may com- 

 mence with the latter as the simplest. It is found in the 

 Medusae known as the Vesiculata. The least differentiated 

 form of this organ, so far discovered, is present in Mitrotrocha, 

 Tiaropsis and other genera. It has the form of an open pit; 

 and a series of such organs are situated along the attached edge 



B. in. 33 



