CEPHALOPODA. 471 



of the internal ear alone exists, imbedded in the as yet cartilagi- 

 nous cranium. Gradually, as in fishes, semicircular canals, pro- 

 longed from the central part, increase the auditory surface, but 

 still have no communication with the exterior of the body. In 

 reptiles and birds destined to perceive sonorous impressions in an 

 aerial medium, a tympanic cavity and drum are superadded ; and 

 lastly, in the Mammiferous orders, external appendages for collect- 

 ing and conveying sound to the parts within, complete the most 

 complex and perfect form of the acoustic instrument. 



As far as is yet known, the Tetrabranchiate Cephalopods have 

 no distinct organ of hearing ; but in the Dibranchiata an ear lodged 

 in an internal cranium for the first time presents itself to our 

 notice, and at the same time exhibits the lowest possible condi- 

 tion of a localized apparatus adapted to receive sounds. 



In the anterior and broadest part of the cartilaginous cranium,* 

 where its walls are thickest and most dense, are excavated two 

 nearly spherical cavities (Jig. SI 5, rf), which in themselves constitute 



Fig. 215. 



the osseous labyrinth of both ears. A vesicle or membranous sac- 

 culus (c), likewise nearly of a spherical form, is suspended in the 

 centre of each of these cartilaginous cells by a great number of 

 filaments that are probably minute vessels. The two auditory 

 nerves derived from the encephalon enter these cavities through 

 special canals ; and each, dividing into two or three branches, spreads 

 out over the vesicle to which it is destined. The auditory vesicle 

 itself is filled with a transparent glairy fluid ; and contains, attached 



* Cuv. MSmoire sur le Poulpe, p. 41. 



