SYSTEMATIC 

 ARRANGEMENT OF BIRDS. 



CLASS XVI. 



AVES. 



VERTEBRATE animals, breathing atmospheric air by means of 

 lungs ; with warm, red blood, and heart biventriculate and biauri- 

 culate; all oviparous, covered with feathers, with bill rather pro- 

 minent, naked, destitute of teeth. Extremities four, the anterior 

 changed into wings, almost always adapted for flying. 



ORDER I. Natatores. 



Feet moderate or short, placed more or less behind (averse), 

 palmate or fisso-palmate. Entire thigh and basal portion of tibia 

 included by the skin of the trunk. 



Surimming-Mrds. They have the inferior part of the tibiae 

 mostly covered with a horny skin, as also the tarsi and toes; legs 

 of this kind are called stilted (pedes vadantes) ; the legs are little 

 longer than the half of the trunk (mediocres), or even shorter than 

 this half (breves), and are placed towards the back part of the 

 trunk (aversi). The plumage is thick, and penetrated as it were 

 with much oily fluid, and thus protected from the water. The 

 neck is often much elongated, so that these birds by extending their 

 head over or under the water seek for and seize their food on 

 every side. The breast-bone is large, more or less convex, and 

 extends over the abdomen far backwards ; it is incised at the pos- 

 terior margin on each side, or provided with an oval opening close 

 to that margin. On the bony or cartilaginous cavities at the 

 inferior extremity of the larynx in some ducks and saw-bills, we 

 have spoken already above (p. 330). 



