II. THE PHYLUM CHORDATA 



A. THE CHARACTERISTICS OF THE CHORDATES 



While the vertebrates comprise the greater part of the phylum Chordata, three small 

 groups of animals are united with them in this phylum because they possess certain char- 

 acteristics in common with the vertebrates. These characteristics are: 



1. The wall of the pharynx of the embryo or adult is pierced by openings, the gill slits, 

 originally for respiratory purposes. 



2. A notochord is present in embryo or adult. The notochord is a rod lying dorsal to the 

 intestine, extending from anterior to posterior end, and serving as a skeletal support. In 

 vertebrates the notochord is partially or wholly replaced by the skull and vertebral column. 



3. The central nervous system is nearly always hollow (in the tunica tes in the embryo 

 only), containing usually a single continuous cavity but in some cases a number of isolated 

 spaces, and is situated entirely on the dorsal side of the body (except in the Hemichordata 

 where there are both dorsal and ventral portions). In the invertebrates, the central nervous 

 system is always solid and lies mainly ventral in the body. 



For further discussion of these characters see P and H, pages 1-2; N, pages 31-32; 

 K, pages 1-2. 



B. THE CHARACTERISTICS OF THE VERTEBRATES 



The morphological characters of the vertebrates are the following: animals with bilateral 

 symmetry, internal heteronomous segmentation, and cephalization; with generally two pairs of 

 paired jointed locomotor appendages, in the form of fins or limbs, and sometimes with unpaired 

 appendages in addition; skin separable from the rest of the body wall and commonly producing 

 protective structures, such as scales, feathers, hair, etc., cellular in nature; muscle layer of the 

 body wall decidedly metameric in arrangement; with an internal skeleton, of cartilage or 

 bone, consisting of a skull and gill supports in the head, vertebral column, ribs, and breast- 

 bone in the body, and supports for the appendages; vertebral column highly metameric, 

 composed of successive rings around the notochord; central nervous system consisting of a 

 brain, much enlarged, within the skull, and a spinal cord within the vertebral column; nerves 

 highly metameric in arrangement; head with three pairs of sense organs, eyes, ears, and nose; 

 digestive tract giving rise by outgrowth to two digestive glands, the liver and the pancreas; 

 pharynx intimately connected with the respiratory system, either opening to the exterior 

 by openings, the gill slits, in the walls of which the gills are borne, or giving rise by outgrowth 

 to the lungs; heart always ventral in the body; circulatory system closed, always with a median 

 dorsal artery, the aorta, and with one or two portal systems; genital and excretory systems 

 closely related, the excretory ducts generally serving as genital ducts; excretory and genital 

 chicts opening in common with the intestine into a cloaca, or opening separately near the anus; 

 with a well-developed coelom, never segmented, and divided in the adult into two or four 

 compartments; viscera supported by mesenteries. (See further, K, pp. 2-4; P and H, 

 p. 119; H, pp. 575-76; Wd, pp. 11-13; and study the diagrams in P and H, p. 69.) 



C. THE CLASSIFICATION OF THE CHORDATES 



Since in this manual reference by their scientific names to groups of chordates is unavoid- 

 able, it is essential that the student learn at once the following scheme of classification. Only 



