SENSE ORGANS. 495 
the spiracle, or first gill-cleft, is situated in the vicinity of 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 the 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 wail 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. 
One interpretation has 
been stated on p. 480; the 
following is Hertwig’s :— 
Malleus = Articular + 
angular elements of 
Meckel’s cartilage. 
Incus = Palato-quad- 
rate of lower Verte- 
brates. 
Stapes of Mammals 
has a double origin, 
being formed from 
the upper part of 
hyoid arch + an ossi- 
fication from the 
wall of the ear cap- 
sule = (wholly?) col- 
umella of Birds, 
Reptiles, and Am- 
phibians. Fic. 264.—Diagram of the eye. 
. C., Cornea; @.4., aqueous humour; ¢.é,, ciliary 
The eye.—There is body; 4, lens; 7, iris; Sc., sclerotic; Ch., 
2 3 choroid; #., retina; v.4., vitreous humour; 
no eye in Amphioxus, y.sp., yellow spot; #., optic nerve. 
it is rarely more than 
larval in Tunicates, it is rudimentary in Myxine and in 
the young lamprey. In higher forms the eye is always 
present, though occasionally degenerate, ¢.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 Cac/za, very small in a few 
snakes and lizards, and its nerves are abortive in the 
mole. 
