XIII 



PHYLUM CHORDATA 



353 



4th of the typical foot, forwards. The entire hind-limb is in a plane 

 parallel with the sagittal plane of the trunk. 



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 a lower eyelid, and a transparent nicti- 

 tating 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 ven- 

 tral surface at the 

 junction of the uro- 

 pygium with the 

 trunk. 



Exoskeleton. 

 The exoskeleton is 

 purely epidermal, 

 like that of the 

 Lizard, which it also 

 resembles in consist- 

 ing partly of horny 

 scales. These cover 

 the tarso-metatarsus 



and the digits of the foot and are quite reptilian in appearance 

 and structure. Each digit of the foot is terminated by a claw 

 which is also a horny product of the epidermis, 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 

 outside the present class. 



A feather (Fig. 964) is an elongated structure consisting of a 

 hollow stalk, the calamus or quill (cat.), 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 



VOL. II A A 



tnf.umb 



FIG. 964. Columba livia. A, proximal portion of a rem ex ; 

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

 sup. umb. superior umbilicus. B, filojjlume. C, nestling- 

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



