xii PHYLUM CHORDATA 457 



entire wing is straightened out at right angles to the trunk. 

 In the hind-limb the short thigh is closely bound to the 

 trunk : the foot is clearly divisible into a proximal portion, 

 the tarso-metatarsus (ts. mtts) and four digits, of which one, 

 the hallux (dg. i), is directed backwards, the others, the 

 2nd, 3rd, and 4th of the typical foot, forwards. 



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 remarkably backward position. The eyes 

 are very large, and each is guarded by an upper and a 

 lower eyelid and a transparent nictitating membrane 

 (net. ;;/.). A short distance behind the eye is the auditory 

 aperture (an. 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 uropy- 

 gium with the trunk. 



The exoskeleton is purely epidermal, like that of the 

 Lizard, which it also resembles in consisting 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, how- 

 ever, is covered by feathers, a unique type of epidermal 

 product found nowhere outside the present class. 



A feather (Fig. 258) is an elongated structure consisting 

 of a hollow stalk, the calamus or quill (cal. ), and an ex- 

 panded distal portion, the vexillum or vane. At the 

 proximal end of the quill is a small aperture, the inferior 



