SENSE OKGANS. . 445 



change in the direction or velocity of movement. How far the ears of 

 lower Vertebrates [e.g-., Crustacea and Molluscs), are adapted for any 

 function except this is still doubtful, and we can hardly see that any 

 other would be of much use to purely aquatic animals. It seems 

 likely at any rate that the primitive function of the ear was the percep- 

 tion of vibrations, and that from this both the sense of hearing, and the 

 sense of equilibration have been differentiated. 



It is in accordance with the facts mentioned above, that we rarely 

 find in Fishes any special path by which impressions of sound may 

 travel from the external world to the ear. In Amphibians and higher 

 Vertebrates, however, the ear has sunk further into the recesses of the 

 skull, and a special path for the sound is present. In Elasmobranchs 

 the spiracle, or first gill cleft, is situated immediately behind the ear ; in 

 higher forms, according to many authors, this first gill cleft is metamor- 

 phosed into the conducting apparatus of the ear. In development a 

 depression beneath the closed gill cleft unites with an outgrowth from 

 the pharynx, and thus forms the tympanic cavity, which communicates 

 with the back of the mouth by the Eustachian tube. The tympanic 

 cavity is closed externally by the drum or tympanum, which may be 

 flush with the surface, as in tlie Frog, or may lie at the end of a narrow 

 passage which in many Mammals is furnished externally with a projec- 

 tion or pinna. In Amphibia and Sauropsida the tympanic cavity is 

 traversed by a bony rod — the columella, which extends from the drum 

 to the fenestra ovalis, a little aperture in the wall of the bony labyrinth. 

 In Mammals this is replaced by a chain of three ossicles, an outermost 

 malleus, a median incus, an internal stapes. 



The homologies of these ossicles are still uncertain. The following 

 is Hertwig's interpretation : — 



Malleus = Articular + angular elements of Meckel's cartilage. 

 Incus = Palato-quadrate of lower Vertebrates. 



Stapes of Mammals has double origin, being formed from the 

 upper part of hyoid arch -1- an ossification from the wall of 

 the ear capsule = (wholly ? ) columella of Birds, Reptiles, and 

 Amphibians. 



TAe Eye. — There is no eye in Amphioxiis, it is rarely 

 more than larval in Tunicates, it is degenerate in Myxine, 

 and in the young lamprey. In higher forms the eye is always 

 present, though occasionally degenerate, e.g., in fishes from 

 caves or from the deep sea. It is hidden under the skin in 

 Proteus, an amphibian cave dweller, and in the subterranean 

 amphibians like Ccecilia, very small in a few snakes and 

 lizards, and abortive as to its nerves in the mole. 



The adult eye is more or less globular, and its walls con- 

 sist of several distinct layers. The innermost layer bounding 

 the posterior part of the globe is the sensitive retina, inner- 

 vated by fine branches from the optic nerve. It may be 

 compared to the nervous matter of the brain, from which, 



