xni 



PHYLUM CHORDATA 



395 



and are incubated or sat upon by the parents for fourteen days, 

 the temperature being in this way kept at about 38 to 40 C. 

 (100 to 103 F.). At the end of incubation the young Bird is 



adr 



r.od- 



FIG. 1060. Columba livia. Male urino- 

 genital organs, adr. adrenal ; c/ 2 ., uro- 

 dseum ; cfi., proctodaeum ; k. kidney ; ts 

 testis, that of the right side displaced 

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

 vas deferens ; vd'. its cloacal aperture 

 r. x. vesicula seminalis. (From Parker's 

 Zootomy.) 



l.od' 



FIG. 1061. Columba livia. Female urino- 

 genital organs, c/ 2 ., urodaeum ; cl 3 ., procto- 

 dseum ; k. kidney ; /. od. left oviduct ; l.od'. 

 its cloacal aperture ; l.od". its ccelomic fun- 

 nel ; l.od'". its ccelomic aperture ; ov. ovary ; 

 r. od. right oviduct ; r. od'. its cloacal aper- 

 ture ; ur. ureter ; ur'. its cloacal aperture. 

 (From Parker's Zootomy.) 



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 Craniata in which the epidermal exoskeleton takes the 

 form of feathers over the greater part of the body, of a rhamphotheca 

 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 

 usually heteroccelous, but may be procoelous or amphiccelous. The 

 sacral vertebras are fused with the lumbar and with more or fewer 

 of the posterior thoracic and anterior caudal to form a syn-sacrum 

 for the support of the ilia. The posterior caudal vertebrae are 

 usually fused to form a pygostyle around which the tail-quills are 

 arranged in a semicircle. The bones of the skull undergo early 

 ankylosis. There is a single, rounded, occipital condyle ; the 



