EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 311 



Reptilia (lizards, snakes, crocodiles, etc.), Class Amphibia (salamanders, 

 newts, frogs, toads), Class Pisces (tishes), and so on. 



Subphylum 



Members of all the classes mentioned above have a few fundamental 

 similarities in common; for example, they all possess a backbone or verte- 

 bral column. Hence they are all grouped together into Subphylum Verte- 

 brata (Fig. 14.1). Vertebrates, members of this subphylum, possess a 

 vertebral column as adults, but during embryonic stages this bony column 

 is preceded by an unsegmented, elastic rod called a notochord (p. 68). A 

 few animals possess a notochord throughout life. These and a few aberrant 

 forms which have a notochord only during larval existence (p. 74; Fig. 5.5, 

 p. 101 ) either are grouped together into one subphylum, or are placed each 

 in a separate subphylum of its own. Thus in Fig. 14.1 we have shown 

 Amphioxus, the form with the persistent notochord, as having its own 

 subphylum: Cephalochorda. 



Phylum 



The subphyla to which we have alluded combine to form Phylum Chor- 

 data (Fig. 14.1); the name refers to possession of a notochord at some 

 time during life. Examples of other phyla are: Phylum Arthropoda (insects, 

 crustaceans, etc.); Phylum Mollusca (snails, clams, oysters, and all kinds 

 of "shell fish"); Phylum Protozoa (one-celled animals, or small animals 

 lacking cellular structure, depending upon one's point of view). 



Ascending Categories 



Thus we see that the classification of the dog consists of an ascending 

 series of more and more inclusive categories (Fig. 14.2). Or the classifica- 

 tion may be looked at from the other point of view, as a descending series 

 of categories, each one subdivided in turn until the smallest unit, the spe- 

 cies, is reached. Fig. 14.2 shows the "steps" involved whether we travel 

 them upward or downward. All of the categories named in the figure are 

 subdivided, thereby increasing the number of "steps" in the series. Thus 

 orders are divided into suborders, families into subfamilies, and so on. This 

 added complexity is not important to the present discussion, however, al- 

 though one subdivision, the subspecies, will receive special attention pres- 

 ently (pp. 320-326). Every animal can be classified in a manner similar to 



