THE PHYLUM CHORDATA 69 



cause they have an amnion, and Allantoida because they 

 have an allantois. 



Class IV. Reptilia reptiles, such as lizards, turtles, croc* 



odiles and snakes. 

 Class V. Aves birds. 



Div. 3 AND CLASS VI. MAMMALIA hairy quadrupeds and other 

 mammals. 



The most significant general subdivision of the Craniata is 

 one that recognizes what was probably the most important 

 evolutionary crisis encountered by the vertebrates: the tran- 

 sition from an aquatic to a terrestrial mode of life. The lower 

 craniates (cyclostomes and fishes) are all aquatic; the Am- 

 phibia constitute a transitional group, but are primarily aquatic, 

 in that some of the degenerate forms are permanently aquatic 

 and all are essentially aquatic during the embryonic and larval 

 periods; all of the higher craniates are terrestrial in the sense that 

 they breathe with lungs instead of with gills. The Ichthyop- 

 sida are all fish-like creatures having the aquatic mode of locomotion 

 and of respiration and a circulation designed for gill respiration. The 

 fish-like form common to the members of the group is to be considered 

 as a general adaptation to aquatic life. They are Anamnia because 

 their eggs develop in water and need no amniotic water-bag to pro- 

 tect the growing embryo. Likewise they are Anallantoida because 

 the allantois is essentially an adaptation for late embryonic respira- 

 tion in the air and is therefore not needed by forms that develop gills 

 early in larval life. 



The air-breathing classes, Reptilia, Aves, and Mammalia, are 

 Amniota and Allantoida for the obvious reason that they are terres- 

 trial in embryonic as well as in adult life. The grouping of the Rep- 

 tilia and Aves into the Sauropsida indicates a conviction prevalent 

 among comparative anatomists that there is an unusually close affin- 

 ity between these two classes. In fact some go so far as to deny class 

 value to the Aves, on the ground that they are merely specialized 

 flying reptiles. The validity of the subdivision into Agnathostomata 

 and Gnathostomata is questioned by some authorities on the ground 

 that the jawless condition of the cyclostomes is said to be due to de- 

 generation, owing to the highly specialized condition of the "rasping 



