A UD I TORY RGA NS. 



423 



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 membranous 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 interesting to notice the 

 presence of a more or less similar membrane in the two great groups 

 of terrestrial forms, i.e. terrestrial Vertebrata and Insecta. 



Nothing is known with reference to the mode of development 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 Coelenterata; 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. 



CcBlenterata. Three distinct tj'pes of auditory organ have been 

 recognised in the Medusae; two of them resulting from the differentia- 

 tion of a tentacle-like organ, and one from ectoderm cells on the under 

 surface of the velum. We may commence 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 dis- 

 covered, is present in Mi- 

 trotrocha, Tiaropsis and 

 other genera. It has the 

 form of an open pit; and 

 a series of such organs 

 are situated ahmg the at- 

 tached edge of the velum 

 with their apertures di- 

 rected downwards. The 

 majority of the cells lin- 

 ing the outer, i.e. peri- 

 pheral side of the pit, con- 

 tain an otolith, while a 

 row of the cells on the 

 inner, i.e. central side, are 

 modified as auditory cells. 

 The auditory cells are 

 somewhat strap-shaped, 

 their inner ends being 

 continuous with the fibres 



Fio. 297. AuBiTORY VESICLE OF Phialibicm after 



TREATMENT WITH DILUTE OSMIC ACID. (FlOm Lail- 



kester; after 0. and E. Hertwig.) 



d}. epithelium of the upper surface of the velum ; 

 d^. epithelium of the under surface of the velum ; r. 

 circular canal at the edge of the velum; nr^. upper 

 nerve-ring; h. auditory cells; hh. auditory hairs; 

 np. nervous cushion formed of a prolongation of the 

 lower nerve-ring. Close to the nerve-ring is seen a 

 cell, shewn as black, containing an otolith. 



