CHORDATA. 321 



DESCRIPTIVE TEXT. 



336. General Characters. In common with the simpler 

 Chordata thus far considered the Vertebrata are bilaterally 

 symmetrical Metazoa with a coelomic cavity, a notochord de- 

 rived from the entoderm, gill-slits at some stage of life, dorsal 

 nerve tube and a ventral heart. In addition, the following 

 points may be given as distinguishing the true vertebrates : 



1. The notochord comes to be surrounded by a sheath of tis- 

 sue derived from the mesoderm. This produces around the 

 notochord the internal skeletal axis, the centra of the vertebrae, 

 composed either of cartilage or bone (Figs. 154-156). 



2. Outgrowths from these centra pass dorsally to protect 

 the nerve tube, and ventrally to protect the viscera (Fig. 157). 



3. Several sets of organs show varying degrees of meta- 

 meric segmentation : e. g., vertebral column ; muscular system ; 

 nervous system. 



4. Jointed appendages having a central skeleton never ex- 

 ceed two pairs ; one pair or both of them may be rudimentary 

 or wanting. 



5. The respiratory system is developed in connection with 

 the anterior end of the digestive tract. 



6. The heart always has as many as two chambers and the 

 blood contains red corpuscles. 



337. General Form. While varying greatly in form, ver- 

 tebrates are typically elongated animals with the mouth at or 

 near the anterior extremity of the long axis. The position 

 of the anus is variable. It may be one half the length of the 

 body from the posterior end. The body is roughly divisible 

 into head and trunk with or without an intervening neck. The 

 neck is more pronounced in the land than in the water forms. 

 Posterior to the trunk containing the body cavity, there may 

 be a tail into which the skeleton is continued but which is 

 destitute of a body cavity. 



Bilateral symmetry is shown by the paired condition of the 

 eyes, ears, and other external and, to a less degree, internal 



