6 VERTEBRATE ANIMALS OF THE UNITED STATES 



derived from it. The vertebrate, on the other hand, in common with 

 all chordate animals, respires directly with the pharynx, the anterior 

 portion of the digestive tract, and with structures derived from the 

 pharynx; the medium in which the animal lives, whether water or air, 

 must be drawn through the mouth into the respiratory organs by an 

 incessently repeated muscular effort, the interruption of which, only for 

 a few moments, will often kill the animal. In fishes the gills are paired 

 lateral pockets of the pharynx which open through the integument to 

 the outside; through these the breathing motions of the animal maintain 

 never ceasing streams of water from which the thin-walled, delicately 

 branched gill filaments absorb the respiratory air. 



In amphibians the larval forms, and to a large extent the adults 

 as well, respire after the manner of invertebrates, by means of the 

 integument. All amphibians, however, in the adult condition, respire 

 also directly with the surface of the pharynx, and also in most species 

 with the lungs, which arise as a ventral diverticulum of it, as well. 

 The importance of direct pharyngeal respiration is shown by the 

 rapid and regular respiratory movements of the throat of salamanders 

 and frogs when in the air, which affect the pharynx alone and do not 

 serve to introduce air into the lungs, and also by the total lack of 

 lungs in a large group of salamanders. Amphibians are the oldest and 

 most primitive terrestrial vertebrates; direct pharyngeal respiration 

 was probably the earliest method among vertebrates of utilizing the 

 atmospheric air for respiratory purposes, and may have grown out of the 

 habit, not uncommon among fishes, of rising to the surface of the water 

 and swallowing air. The lungs, in the earliest amphibians, were prob- 

 ably primarily hydrostatic organs like the air-bladder of fishes, a 

 function they still retain in many primitive salamanders, and to a cer- 

 tain extent probably in all amphibians. 



In the amniotic vertebrates respiration is exercised by the lungs 

 exclusively rather than by the pharynx directly, and the complexity 

 of their structure in each group of such animals bears a direct relation 

 to the intensity of the metabolism in that group. The cold-blooded, 

 sluggish reptiles have sack-like lungs with relatively large and relatively 

 few lung vesicles. The lungs of the warm-blooded vertebrates, on the 

 other hand, which maintain a constant temperature in their bodies 

 without reference to the surrounding medium, contain a dense mass of 

 minute vesicles which represent a very much larger relative absorptive 

 surface and gives the lungs the spongy texture characteristic of them, 

 the birds with their higher temperature possessing lungs with smaller 

 vesicles and consequently a larger relative surface than the mammals. 



