390 Comparative Morphology of Chordates 



The basic definition of a group must include all of the character- 

 istics which have been taken into account in estimating the genetic 

 position of the animals to be included in it. However, for purposes of a 

 condensed summary of a classification, it is convenient to devise very 

 brief descriptive designations using perhaps only one or two of the 

 most salient features of a group. Modern birds may be safely dis- 

 tinguished as vertebrates having feathers, but birds have many other 

 characteristics which are quite as distinctive as feathers. 



The animal kingdom is usually divided into some 12 to 14 groups 

 distinguished from one another by morphologic differences of major 

 importance. Each of these groups is called a Phylum. The Phylum is 

 subdivided into a series of groups whose distinguishing characteristics 

 are of successively lower orders of importance. Named in descending 

 sequence, the series includes Classes, Orders, Families, Genera, and 

 Species. Sometimes it is found convenient to make a subphylum, sub- 

 class, or suborder. Occasionally two or more groups are collected to- 

 gether under some descriptive heading for which there is no special 

 taxonomic name. The terms Anamnia, Amniota, Sauropsida, and 

 others used in classifying vertebrates, are of this unnamed sort. They 

 are referred to informally as "groups" or "divisions." 



The scientific name of the individual animal is the Linnaean bi- 

 nomial (see p. 340), consisting of the Genus and Species to which the 

 animal belongs. The common cat is Felis domestica; the lion, Felis 

 leo; the tiger, Felis tigris; and there are, in all, some 40 species more 

 in the genus Felis. Linnaeus called man Homo sapiens. We belong to 

 the family Hominidae (of which we are the only living species), to 

 the order Primates, the class Mammalia, the phylum Chordata. 



The names of the several phyla which are commonly recognized 

 are shown in the hypothetical "tree" (Fig. 301). The name, Chordata, 

 of the phylum to which the vertebrates belong does not appear in the 

 figure. The Chordata are shown as divided into their several subphyla 

 and classes, comprising all of the right trunk of the tree above the 

 level of the Echinoderms. 



The phylum Protozoa occupies a unique position. Its members are 

 those animals which are so small (most of them being of microscopic 

 dimensions) that the protoplasmic body is organized as a single cell — 

 the "unicellular animals." The members of all the other phyla, with a 

 few exceptions, are relatively large, and their living substance is there- 

 fore organized into numerous cells, because the necessities of proto- 

 plasmic metabolism impose limits on the maximum volume of a cell. 

 All the phyla aside from Protozoa are referred to collectively as the 

 Metazoa, the "multicellular animals." It is generally believed that 



