6 STRUCTURE OF THE VERTEBRATES 



ventral in position, and blood from the heart is forced dorsally 

 and then posteriorly, the large veins returning the blood being 

 ventrally located. 



Nomenclature. Scientific terms can best be explained when 

 they first appear in the text. In addition, a glossary is appended 

 to assist the student when these definitions have been forgotten. 

 A few terms, however, which involve evolutionary principles, are 

 defined here before the classification of the phylum is given. 



1. Primitive, when used to app'ly to organisms, refers to that 

 which is racially ancient. Technically, any animal which is liv- 

 ing today is "recent"; but if it retains many of the character- 

 istics which were typical of its early ancestors, it is referred to 

 as a primitive animal. 



2. Specialized animals are those which, during the course of 

 evolution, have become adapted to life in a limited environment; 

 or those which are specialized for limited activity. The entire 

 animal may be spoken of as specialized, or one may refer to 

 specialized individual structures. Therefore an animal may be 

 primitive, and yet specialized in many of its characteristics. 



3. Generalized animals or characters are those which are 

 capable of modification, those which are not specialized toward 

 one particular function. ]\Ian's hand is more generalized than 

 his foot. 



It is clear that any one of these terms may be applied to an 

 individual structure, or to an animal as a representative of a 

 group. If the latter, the entire sum of characters must be con- 

 sidered. Consequently, a generalized animal may have many 

 specialized characteristics; and a primitive animal may be 

 either generalized or specialized. Further, each of the three terms 

 is used as a limiting adjective, and any statement must be 

 applied to a specific group and considered in relation to other 

 similar races. We may speak of a primitive man, though he 

 would not be a primitive vertebrate; or, a primitive vertebrate 

 would not be primitive in relation to the lower chordates. Limit- 

 ing the term thus is necessary due to the very nature of the 

 evolutionary process. For, during millions of years of life, it is 

 inconceivable that any large group of animals could have existed 

 without changes and specializations; and these changes would 



