36o CHORD AT A. 



CHAPTER XXII. 



CHORDA TA^( 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 head is well separated from the trunk by a long 

 and flexible neck and at the hind-end of the trunk there is 

 a small and stumpy tail. The beak 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 cere. 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 



