xm 



PHYLUM CHORDATA 



369 



rcli 



The mouth is terminal, and is guarded by the elongated upper 

 and lower beaks ; it has, therefore, a very wide gape. On each 

 side of the base of the upper beak is a swollen area of soft skin, 

 the cere (cr.), surrounding the nostril (na.), which has thus a remark- 

 ably backward position. The eyes are very large, and each is 

 guarded by an upper and lower eyelid and a transparent nictitating 

 membrane (net. m.). A short distance behind the eye is the auditory 

 aperture (au. ap.), concealed by feathers in the entire Bird, and 

 leading into a short external auditory meatus, closed below by the 

 tympanic membrane. The anus or cloacal aperture (an.) is a large, 

 transversely-elongated aperture placed on the ventral surface at 

 the junction of the uropygium with the trunk. 



Exoskeleton. The exoskeleton is purely epidermal, like that 

 of the Lizard, which 

 it also resembles in 

 consisting partly of 

 horny scales. These 

 cover the tarso-meta- 

 tarsus and the digits 

 of the foot, and are 

 quite reptilian in 

 appearance and 

 structure. Each digit 

 of the foot is termi- 

 nated by a claw, 

 which is also a horny 

 product of the epi- 

 dermis ; and the 

 beaks are of the same 

 nature. The rest of 

 the body, however, 

 is covered by feathers, 

 a unique type of 

 epidermal product 

 found nowhere out- 

 side the present class. 



A feather (Fig. 

 1034) is an elongated 

 structure consisting 

 of a hollow stalk, the calamus or quill (cal.), and an expanded distal 

 portion, the vexillum or vane. At the proximal end of the quill is 

 a small aperture, the inferior umbilicus (inf. umb.), into which fits, 

 in the entire Bird, a small conical prolongation of the skin, the 

 feather papilla. A second, extremely minute aperture, the superior 

 umbilicus (sup. umb.), occurs at the junction of the quill with the 

 vane on the inner or ventral face of the feather, i.e., the face adjacent 

 to the body. A small tuft of down in the neighbourhood of the 



inf. umb 



Fid. 1034. Columba livia. A, proximal portion of a 

 remex. cal. calamus ; inf. umb. inferior umbilicus ; rch. 

 rachis ; sap. umb. superior umbilicus. B, filoplume 

 C, nestling-down. (C. from Bronn's Thierreich.) 



