410 Comparative Morphology of Chordates 



other vertebrates, having a transversely elongated mouth with upper 

 and lower jaws, are collectively called Gnathostomata. In contrast 

 to the four-finned fishes, the Amphibia and Amniota, whose members 

 are, in general, four-legged, constitute the Tetrapoda. Even birds are 

 tetrapods because the internal structure of the wing is, in a modified 

 way, like that of a foreleg. The hoatzin (Opislhocomus, a South Ameri- 

 can bird), before it acquires the power of flight, uses the wings in 

 climbing and crawling about. The terms Ichthyopsida and Saurop- 

 sida, devised by T. H. Huxley, are often convenient. "Ichthyopsida" 

 means "fishlike." Authors differ as to whether it should or should not 

 include the cyclostomes. They are included in it here on the ground 

 that they are as fishlike as are the frogs and toads. Birds are conspicu- 

 ously reptilian in many respects. Therefore "Sauropsida" is a useful 

 term comprising reptiles and birds. (A somewhat different use of 

 "Sauropsida" has been recently proposed: see p. 629.) Finally, a 

 highly important distinction is that based on the temperature of the 

 body. Some vertebrates are "cold-blooded," others are "warm- 

 blooded." But these terms are inaccurate. "Cold-blooded" is intended 

 to designate that condition in which the animal's temperature approxi- 

 mates that of the external medium and varies with changes of external 

 temperature. Animals having such variable temperature are "poikilo- 

 thermous." In "warm-blooded" animals the internal temperature 

 is stabilized at a particular level independently of changes in external 

 temperature — the "homothermous" condition, characteristic of 

 birds and mammals only. 



Conveniently brief definitions of the several Classes follow. 



Cyclostomata: round-mouthed eels. 



Pisces: permanent gills; paired appendages ar^e fins. 



Amphibia: gills either temporary or permanent or (rarely) lacking; 

 jointed pentadactyl legs. 



Reptilia: poikilothermous amniotes; dry skin with thick external 

 horny layer differentiated into scales. 



Aves: feathers. 



Mammalia: hair, milk (mammary) glands. 



In the following accounts of the several Classes, more extended 

 descriptions will be given and the larger subdivisions of each class will 

 be described. 



Geologic Times 



In the following chapters, occasional reference will be made to the 

 ancient history of groups of vertebrates. Readers who may be un- 

 familiar with the names and sequence of the more or less arbitrary 



