380 



ZOOLOGY 



SECT. 



various glands ; first with layers of albumen or " white," next with 

 a parchment-like shell-membrane, and lastly with a white calcareous 

 shell. They are laid, two at a time, in a rough nest, and are incu- 

 bated or sat upon by the parents for fourteen days, the temperature 

 being in this way kept at about 40 C. (104 F.). A$ the end of 



r.d. 



elf 



M-7* 



r.od- 



alf 



l.od,' 



FIG. 990. Columba livia. Male urino- 

 genital organs, adr. adrenal ; cl. 3, uro- 

 daeum ; cl. 3, proctodfoum ; k. kidney ; ts. 

 testis, that of the right side displaced ; 

 ur. ureter ; ur'. aperture of ureter ; vd. vas 

 deferens ; vd'. its cloacal aperture ; v. s. 

 vesicula seminalis. (Frona Parker's Zoo- 

 tomy.) 



FIG. 991. Columba livia. Female urino- 

 genital organs, cl. S, urodjeum ; cl. -1, procto- 

 dseum ; k. kidney ; 1. od. left oviduct ; I. od'. 

 its cloacal aperture ; 1. od". its ccelomic funnel; 

 1. od"'. its ccelomic aperture ; or. ovary ; r. od. 

 right oviduct : r. od'. its cloacal aperture ; 

 ur. ureter; ur'. its cloacal aperture. (From 

 Parker's Zootomy.) 



incubation the young Bird is sufficiently developed to break the 

 shell and begin free life. It is at first covered with fine down, and 

 is fed by the parents with a secretion from the crop, the so-called 

 " Pigeon's milk." 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



Aves are Cramata in which the epidermal exoskeleton takes the 

 form of feathers over the greater part of the body, of a rhampho- 

 theca or horny sheath to the beak, and of claws on the digits of the 

 foot and sometimes of the hand. In the standing position the 

 body is entirely supported on the hind limbs, the articulations of 

 which are thrown forward. The fore-limbs are modified to form 

 wings, usually provided with large feathers for the support of the 

 body during flight. The cervical and free thoracic vertebrae are 



