-b'O ORGANS OF THE SENSES. 



tions transmitted to this part of the head, the organs of 

 hearing being here enclosed in the thickest and densest 

 part of the great cephalic curved plate into which the strong 

 muscles of the arms are inserted. In the octopus ven- 

 tricosus, this cretaceous body is conical like a limpet, of 

 a rose-red colour on its round exterior tapering surface, 

 white and hollow on the base which rests on the vestibular 

 membrane, and it is connected, as in higher animals, to the 

 side of the vestibule by numerous nervous filaments. The 

 ento- and peri-acoustic lymph seen here in the vestibule 

 corresponds with that found in the entomoid articulata and 

 in the ears of vertebrated animals, and notwithstanding the 

 magnitude and constancy of the auditory organs in the 

 naked cephalopods, and their presenting the most complex 

 form met with in the invertebrated classes, they are yet 

 reduced almost to the first rudiment or most essential 

 element of the auditory apparatus the vestibular sac with 

 its acoustic nerve. 



In tracing this organ upwards through the vertebrated 

 classes we find it gradually perfecting the semi-circulajr canals, 

 developing a cochlea from the vestibule, and enveloping the 

 whole of this complex labyrinth within the solid texture of 

 the cranium. It acquires in the air-breathing animals a 

 tympanic cavity communicating with the fauces by the 

 Eustachian tube, and containing ossicula auditus which 

 transmit the vibrations of the membrana tympani to the 

 vestibule and the whole internal labyrinth. And in the 

 highest conditions of the organ a still more exterior meatus 

 auditorius and complicated moveable concha are added to 

 complete this acoustic instrument. Fishes, like the cepha- 

 lopods, receive their auditory sensations through the dense 

 watery element they inhabit, the undulations of which strike 

 forcibly the whole surface of their head and trunk, so that 

 they less require any external means of concentrating sono- 

 rous vibrations or a complicated internal auditory apparatus, 

 than animals which hear through a thin and rare aerial 

 medium. In the lowest cyclostome cartilaginous fishes, as 

 the lamprey, the whole internal ear is nearly in the same 

 condition as in the cephalopods, consisting of a simple shut 

 vestibule, without fenestra ovalis, enclosed in the cartilagi- 

 nous substance of the cranium, without internal calcareous 



