INTRODUCTION. 3 



Frog traces its life-history from the egg to the adult condition, 

 and is separated from the two preceding branches merely for the 

 sake of convenience. In the solution of many biological pro- 

 blems it is often of the greatest possible use. An animal in the 

 course of its development passes through a series of stages which 

 to some extent indicate its pedigree, or, as it has been put, 

 "climbs up its own genealogical tree." Thus the Frog starts 

 active life as a gill-breathing aquatic tadpole which is practically 

 a fish in structure, and points to descent from fish-like ancestors. 

 (4) Classification refers the Frog to a definite place in a 

 system of grouping, in which animals are best arranged according 

 to those characters which denote blood-relationship. It is found 

 possible to divide the animal kingdom into several large branches 

 or phyla, which are again subdivided, and the process continued 

 until the individual is reached. The relative status of the various 

 groups may be indicated as follows : 



PHYLUM 



SUB-PHYLUM 



CLASS 



ORDER 

 Family 



Genus 



Species 



Variety. 



The Frog is placed in the important phylum CHORDATA, which 

 includes all animals that possess, temporarily or permanently, an 

 elastic supporting rod, the notochord, below the central nervous 

 system, as well as clefts which place the cavity of the throat in 

 communication with the exterior. Mammals, Birds, Eeptiles, 

 Frogs, Fishes, and certain somewhat lower forms are here included. 

 The Frog's sub-phylum, that of the VEETEBEATA, embraces 

 animals among the common characters of which are possession 

 of a more or less complete brain-case and spinal column. It 

 includes the above-mentioned animals except the forms lower 

 than fishes. Vertebrates are subdivided into several classes, one 

 of which, the AMPHIBIA, comprises frogs, toads, newts, sala- 

 manders, and the like, all of which pass through a tadpole-stage. 



