THE ANIMAL KINCiDOM 3 



is populifolia, because it has leaves that twinkle in the wind like those of a 

 poplar tree. 



The common cat is Felis domestica; the lion, Felis leo; the tiger, 

 Felis tigris; and there are, in all, some forty species more in the genus 

 Felis. Linne called us Homo sapiens. We belong to the family Homi- 

 nidae (of which we are the only living species), to the order Primates, the 

 class Mammalia, the phylum Chordata, and the animal kingdom. In a 

 general way, for the larger and more famiHar animals and plants, the 

 vernacular name, such as pine or elephant, refers to the genus. 



On the basis of the number of cells in the body animals are divided into 

 the two Sub-Kingdoms — (i) the unicellular Protozoa, (2) multicellular 

 Metazoa. Since the Protozoa are the simpler organisms, it may be 

 assumed that the first animals on earth were protozoans. 



The division of animals into the two phylogenetic series graphically 

 represented in Fig. i is based upon differences in the fate of the embryonic 

 mouth or "blastopore." The left-hand branch includes the Protero- 

 stomians, which are the Metazoa in which the blastopore becomes the 

 mouth or lies near the adult mouth. Most animal phyla are Protero- 

 stomians, which include such diverse forms as the Porifera, Coelenterates, 

 Platyhelminths, Molluscoids, Rotifers, Annelids, Molluscs and Arthro- 

 pods. This branch of the animal kingdom reaches its climax in the 

 arthropods and molluscs. 



The Deuterostomians are the animals in which the blastopore becomes 

 the anus or lies near the anus. The group includes the Echinoderms and 

 the Chordates. The branch reaches its cUmax in the vertebrates and man. 



Since our present interest centers in man and chordates, and since 

 none of the non-chordate phyla are believed to lie in the direct line of 

 human ancestry, the non-chordate phyla will be mentioned only when 

 they possess structures resembUng those of the hypothetical ancestors of 

 man. It may be assumed that students in a comparative anatomy course 

 have some acquaintance with non-chordates. Consequently no detailed 

 description of them is needed here. We may therefore turn our attention 

 directly to the sub-phyla and classes of chordates. 



Phylum CHORDATA 



The chordates are animals which, at least early in life, have a supporting 

 rod, the notochord or chorda dorsalis, between the alimentary canal and 

 the central nervous system. In higher chordates the notochord is replaced 

 during ontogenesis by a cartilaginous or bony vertebral column. All have 

 a dorsal tubular nervous system. The heart is ventral, and the pharynx 

 has functional or embryonic gill sUts. Most chordates are metameric in 

 structure, although the metamerism may become greatly obscured in the 

 adult. Segmental excretory organs are generally present. 



