Introduction 



The basic structure common to all vertebrate chordates has been 

 described in Part I. This common structure, however, is highly suscep- 

 tible to adaptive modification. The function of Part III is threefold: 

 (1) to describe the major structural adaptations related to the animal's 

 habitat and mode of living; (2) to compare the more conspicuous and 

 functionally important modifications; (3) to classify the chordates in 

 accord with the principles set forth in Part II. 



In Part III, therefore, knowledge of the basic structure is as- 

 sumed and the descriptive material is, in the main, restricted to 

 those specialized structural features which enable us to distinguish 

 the more important subdivisions (subphyla, classes, orders) of the 

 Phylum Chordata. Organs and systems, the nature or degree of whose 

 specialization is not especially significant for the group under considera- 

 tion, may receive only brief mention or may be quite omitted. In some 

 instances, extinct vertebrates are mentioned. Occasional necessarily 

 brief and very general references to important epochs in the past of 

 vertebrates may serve to give the reader some fragmentary glimpses 

 of the tremendously dramatic history of the group. The classification 

 is restricted to the now living groups. Collateral reading in paleon- 

 tology will be helpful. 



