XII PHYLUM CHORDATA 459 



a large gland, the oil-gland (o.gl), used for lubricating or 

 " preening " the feathers. 



The wings show the three typical divisions of the fore- 

 limb, upper arm, fore-arm, and hand, but the parts of the 

 hand are closely bound together by skin, and only three 

 imperfectly marked digits, the second (dg 2) much larger 

 than the first (dg 1) and third (dg 3), can be distinguished. 

 In the position of rest the three divisions of the wing are 

 bent upon one another in the form of a Z : during flight the 

 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 its. mtts) and four digits, of which one, 

 the hallux (dg /'), is directed backwards, the others, the 

 second, third, and fourth 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 gap. 

 On each side of the base of the upper beak is a swollen 

 area of soft skin, the cere (cr) surrounding the nostril (no), 

 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. ni). A 

 short distance behind the eye is the auditory aperture 

 (au. ap), concealed by feathers in the entire bird, and lead- 

 ing into a short external auditory meatus, closed below 

 by the tympanic membrane. The anus or cloacal aper- 

 ture (an), is a large, transversely elongated aperture placed 

 on the ventral surface at the junction of the uropygium 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 die digits of 

 the foot, and are quite reptilian in appearance and structure. 



