412 CHORDATA. 
In the pigeon a further complication is involved in the 
formation of the outer ear, represented by an external audi- 
tory meatus leading from the exterior to the tympanum ; 
and, lastly, in the rabbit, the Azza is added. 
In the mammals the columella appears to be represented 
by three auditory ossicles, as noticed in the rabbit. 
These three sense-organs, their accessories, and the brain 
mark’ out the head of the Vertebrata. 
Fig. 298.—A DIAGRAM OF THE VERTEBRATE EAr. 
Semi-circular 
Canals. 
,Ductus Endo- 
lymphaticus. 
External Audi- 
tory Meatus. y 
vo 
as) Bony Labyrinth. 
Tympanum. /f & DB 3g Perilymph. 
7 ES 
@ & 8 
m@ O09 
wm 
2 ees 
Tee 
55 28 
Ae a? 
The whole diagram represents the ear of the rabbit (except that only one 
ear-ossicle is indicated) ; all to the right of AA represents the ear of the pigeon ; 
to the right of BB represents the frog with middle and inner ear only; and the 
ear of the skate is represented by the part to the right of CC, forming the inner 
ear only. 
Skeletal Organs.—The skeleton of Vertebrata shows 
a succession of three kinds, which replace each other in time 
throughout the classes and in the development of the higher 
