VERTEBHATA. 211 



The liver is two-lobed, rarely there is also a small central lobe. 

 A gall-bladder may or may not be present, even in allied 

 species, or occasionally in the same species. The kidneys are 

 remarkable for being placed in cavities of the iliac bones, as in 

 reptiles, and their substance is homogeneous. The lungs are 

 attached to the ribs and spine, but they are al first free. The 

 diaphragm is not well developed. 



Birds have no eyelashes, but they possess a third eyelid [mem- 

 brana nictitans], transparent or otherwise. The sclprotica ante- 

 riorly is supported by covered bony plates. There is always an 

 ear, but no external concha. In no bird is the eye ever rudi- 

 mentary, or wanting. The brain is very variable in size ; it is 

 about ^V the weight of the body in the sparrow, and TT Vcy in the 

 ostrich. The optic thalami are small, and the optic lobes, dif- 

 fering from " every other class," are lateral and inferior. 



The feathers in birds consist in general of the shaft [scapus], 

 a continuation of the quill [calamus], to which the " webs " are 

 attached on each side ; these are made up of a number of barbs 

 [rami], each furnished with hooked branches [barbules or ramuli]. 

 The "contour-feathers" [pennse], as distinguished from "down- 

 feathers" [plumje], are worked by muscles, sometimes too small 

 to be easily detected, but amounting to four or five to each 

 feather. In Anas marilci and in Sula bassana, Nitzsch counted 

 3000 of the contour-feathers, so that in each bird there must 

 have been 12000 of these muscles. 



The down on newly hatched birds is only partial and tem- 

 porary. The dry membranous bodies in the quill are the remains 

 of the pulpy medulla of early life. Once or twice a year birds 

 moult or renew their feathers; in many cases the difference 

 between the summer and winter moults is very great. 



In many groups the female is much less or very differently 

 coloured to the male, the young birds are mostly coloured like 

 the females ; in the latter it is probably due to the arrested deve- 

 lopment 'of the colouring-matter, just as, on the other hand, old 

 female birds sometimes partially assume the coloration of the 

 male. 



Peculiar formations of integument [corium] are known as 

 caruncles, wattles, &c. The cere at the base of the bill is of the 

 same nature. 



All birds are oviparous ; they have usually only one ovary, 

 which lies towards the left side. The period of incubation varies 

 from two weeks to nearly two months. 



As to the nests of birds, Wallace draws the conclusion that 

 " when both sexes are of strikingly gay and conspicuous colours. 



