Classification of Chordates: Protochordata 409 



of the other group. The great majority of the Anamnia are aquatic and 

 most of these are lifelong gill-breathers. Some (amphibians) have gills 

 only in a temporary larval stage. A few amphibians omit the larval 

 stage and never have gills. The paired appendages are fins in most of 

 the Anamnia, but in some (amphibians) are legs; and the skin is always 

 mucous, even in the land-dwellers. The great majority of the Amniota 

 are terrestrial. All are lung-breathers, whether living on land or in 

 water, and never at any time possess gills. The skin does not produce 

 mucus. The paired appendages are structurally of the land type, even 

 in the aquatic members of the group. In short, despite the apparently 

 anomalous occurrence of lungs and legs in water-dwellers and a mucous 

 skin in a few land-dwellers, the distinction between Anamnia and 

 Amniota seems to be sound and in full accord with all evidence bearing 

 on the genetic relationships of the groups. 



The next step in the classification is comparatively easy. The 

 members of each of the primary divisions, Anamnia and Amniota, 

 readily fall into several groups which, so far as recent vertebrates are 

 concerned, are sharply marked and readily defined — fishes, amphib- 

 ians, reptiles, birds, and mammals. Among "fishes," however, are a 

 few which are so very unlike the rest that it seems best to separate 

 I hem. These are the "round-mouthed" eels. Accordingly, it is the 

 common practice to recognize six Classes under Vertebrata as follows: 

 Cyclostomata, Pisces, Amphibia, Reptilia, Aves, Mammalia. 

 The first three classes are anamniote; the second three are amniote. 

 The following classification includes only groups which contain living 

 representatives. Inclusion of extinct groups would greatly extend and 

 complicate the classification. Certain extinct groups which are im- 

 portant as "connecting links" will be briefly mentioned. The general 

 scheme of the major subdivisions of Vertebrata is as follows: 



Subphylum Vertebrata 

 division anamnia 



aonatha Class Cyclostomala 



Class Pisces J Vkhthyopsida 



Class Amphibia \ / Poikilothermous 



• DIVISION AMNIOTA 



o T ,i,,i T . \ / Class Reptilia 



STOMATA ] poDA \ V x S-SAUROPSIDA 



I Class Aves 



Class Mammalia > Homothermous 



The cyclostomes have a round suctorial mouth devoid of jaws. 

 Therefore, in contrast to the jawless (agnathous) Cyclostomata, all 



