360 CHORDATA. 
CHAPTER XXII. 
CHORDATA—( Continued.) 
V.—COLUMBA. 
PHYLUM CHORDATA (p. 402). 
SuB-PHYLUM VERTEBRATA (p. 405). 
CLAss - AVES (p. 447). 
Columba livia (the common Pigeon) is a type of con- 
venient size for illustrating the important class of Aves or 
birds. It shares with the next type, the rabbit, the fate of 
domestication by man. As explained in Chapter X., a 
careful selection of suitable varieties by man has led to the, 
production of numerous breeds, such as fantails, pouters, 
jacobins, &c., which, especially in external characters, may 
differ remarkably from each other. If a number of these 
breeds be left together and allowed to breed promiscuously, 
the offspring rapidly reverts to the common wild pigeon from 
which they have all been derived. Our description will apply 
to any domestic pigeon. 
The ead is well separated from the ¢runk by a long 
and flexible eck and at the hind-end of the trunk there is 
a small and stumpy ¢az/. The deak is formed of horny 
material covering both jaws. At its base is a small 
pair of external nares, often surrounded by a sensitive 
swollen patch of skin called the ceve. The eyes are large and 
have upper and lower eyelids. In addition, there is a thin 
membranous eyelid which can be drawn from the anterior 
angle transversely across the eyeball. It is called the 
nictating membrane. 
A little way behind and below the eye is a round hole or 
aperture leading into a tube, the external auditory meatus. 
This passes in for some distance and terminates in the drum 
or tympanum. Hence the tympanum in the bird is not at 
the surface, as in the frog, but is sunk to the base of a 
meatus or canal. The mouth opens between the jaws 
into a buccal cavity, on the floor of which is a pointed 
