408 Comparative Morphology of Chordates 



ing is by lungs, and either the skin is dry or, if glandular, the secretion 

 is not mucus. 



According to the findings of paleontology, the earliest vertebrates 

 were aquatic and fishlike. Land vertebrates appeared very much later. 

 It would seem, therefore, that in the attempt to classify vertebrates 

 the first step would be very simple. It could be expected that a line 

 might be drawn separating them into two groups: on one side, aquatic 

 animals with fins, gills, and mucous skin; on the other side, land ani- 

 mals with legs, lungs, and nonmucous skin. But in reality it is not so 

 simple. There are a few animals (some amphibians) which have land 

 legs but pass their lives in the water. There are many which breathe 

 only by lungs, yet live in the water (sea turtles, whales, porpoises). 

 Some fishes have lungs. The "mud puppy" (Nediirus, an amphibian) 

 has a combination of legs, lungs, gills, and a mucous skin, and lives all 

 the time in the water. Further, there are some amphibians which have 

 legs and lungs and live on land, but have a mucous skin and otherwise 

 are anatomically much more like fishes than like typical land animals. 

 And there are many vertebrates which are highly adapted to aquatic 

 life and yet, in their general anatomy, are like land vertebrates. In 

 short, on the basis of habitat and adult organs directly related thereto, 

 it is very difficult, if not impossible, to make any satisfactory primary 

 subdivision of the vertebrates. The study of the embryonic develop- 

 ment of vertebrates, however, has revealed something which proves 

 very significant as a basis for classification. 



In the development of some vertebrates (e.g., turtle, bird, cat), the 

 embryo becomes enclosed within a system of thin membranes (Figs. 

 239, 241) which are produced by the embryo and are composed of 

 living cellular material. In certain of these membranes (allantois and 

 yolk-sac) there are blood-vessels through which the embryonic blood 

 circulates. This system of membranes provides for the mechanical 

 protection, the respiration, and, to varying extents, the nutrition of 

 the developing animal. The membrane which is next to the body of the 

 embryo is the amnion. In other vertebrates (e.g., fish, frog), no such 

 membranes are produced by the embryo, which therefore usually 

 develops in direct contact with the external medium, water. 



If, now, the vertebrates are separated into two groups, one, Amni- 

 ota, including those whose embryos have the amnion and the other 

 membranes which are always associated with it, and the other group, 

 Anamnia (or Anamniota), including those in which no such mem- 

 branes are produced, the result is perfectly satisfactory as the initial 

 step in classifying vertebrates. The adult members of each group have 

 a high degree of anatomic similarity. However great their differences, 

 the members of one group are more like one another than like members 



