206 



FIRST LESSONS IN ZOOLOGY. 



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The senses of sight, smell, and hearing are acute in birds, 

 particularly that of sight. Birds have three eyelids, besides 



an upper and lower lid, a mem- 

 brane which can be drawn over 

 the eye, and is called the "nic- 

 titating membrane/' This cov- 

 ers the whole front of the eye- 

 ball like a curtain. With this 

 it is said the eagle can look di- 

 rectly at the sun. The eyes of 

 owls, hawks, and eagles are pro- 

 vided with a ring of bony plates 

 occupying the front of the 

 sclerotic. By means of this 

 ring the eye can adjust itself 

 like a telescope so as to take in 

 both near and distant objects. 

 The penguin also is endowed 

 with this apparatus, which ena- 

 bles it to adjust its eye to see 

 both above and under the water. 

 Though birds (except the 

 night-birds, especially the owls) 

 have no external ear, they can 

 . hear well; otherwise what would 



FIG. 216. Digestive canal of a seed - 



eating bird OP, oesophagus; cr, be the US6 01 their powers of 

 crop: pv. proventriculus : gz, giz- 



zard: 7, liver: /, pancreas: cce, SOng f 



caecum: x/, small intestine: li, ml , . -. .,, 



large intestine: ow, oviduct; u, Hie 6ggS OI birds are, With 



the exception of those of lizards, 



enormous in proportion to those of other vertebrate ani- 

 mals. The largest egg known is that of the ^Epyornis, an 

 extinct bird of Madagascar, which is about a third of a metre 

 (13| inches) in length. Birds lay but few eggs, and the 

 young of those which build nests are, when hatched, blind, 

 naked, unable to walk, and are fed by the parent birds. In 

 the fowls, such as the hen and partridge, as well as the 

 ducks and other swimming birds, the young, on breaking 



