MAMMALS. 393 



auricle and a ventricle. In air-breathing vertebrates the 

 auricle is, and the ventricle may be, divided. 



8. The sexes are usually separate, and reproduction by 

 budding, etc., is unknown. 



9. The Vertebrata are divided into the Cvclostomata 



V 



and the Gnathostomata. 



10. The CYCLOSTOMATA lack true jaws and paired ap- 

 pendages; they have a single nostril. Some are parasitic. 



11. The GNATHOSTOMATA have true jaws and paired 

 nostrils. 



They are subdivided into Pisces, Amphibia, Reptilia, 

 Aves, and Mammalia. 



12. The Pisces or Fishes have median and usually paired 

 fins; they usually have scales; they breathe by gills and 

 have a two-chambered heart. 



13. The Pisces are divided into Elasmobranchii, Ga- 

 noidei, Teleostei, and Dipnoi. 



14. The Amphibia usually have true feet; they have 

 lungs, and in the young gills as well. A metamorphosis is 

 common. The heart is three-chambered. 



15. The Amphibia are divided into the Caecilia, Urodela, 

 and Anura. 



16. Pisces and Amphibia are called Ichthyopsida on 

 account of their gills and their aquatic life. 



17. Reptilia have external scales and a three- or four- 

 chambered heart; they lack functional gills at all times. 



18. Recent Reptiles are grouped as Lacertilia, Ophidia, 

 Testudinata, and Crocodilia. 



19. Aves or Birds are characterized by the presence of 

 feathers, a four-chambered heart, warm-blood, and wings. 



20. Birds and Reptiles are grouped as Sauropsida on 

 account of the single occipital condyle, the similar scales and 

 the large eggs. 



