324 ZOOLOGY 



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

 composed either of cartilage or bone (Figs. 156-158). 



2. Outgrowths from these centra pass dorsally to protect 

 the nerve tube, and ventrally to protect the viscera (Fig. 159). 



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. 



345. 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 

 organs. Metamerism on the contrary is much more evident 

 from the internal than from the external organs. There are 

 usually two pairs of lateral appendages for support and loco- 

 motion: the thoracic at the anterior end of the trunk, and the 

 pelvic, ordinarily occurring near the union of the trunk and 

 tail. These are variously modified as to their form and in- 

 ternal structure (e.g., fins, legs, arms, wings), but are looked 

 upon as homologous. In many water forms there are median 

 appendages (dorsal, ventral, and caudal fins) also assisting in 

 locomotion. The ccelom or body cavity is well represented in 

 the trunk region, and arises by a splitting of the mesoderm into 

 an inner layer which comes to unite with the digestive tract 



