TEXT-BOOK OF ZOOLOGY 



DIVISION OR TYPE: I. VERTEBRATES. 



Bilaterally symmetrical animals with an internal bony or cartilaginous 

 skeleton. On the dorsal side of the vertebral column lies the central 

 nervous system (brain and spinal cord) ; on the ventral side lie the 

 organs of respiration, circulation and digestion. Usually four limbs, 

 rarely two or none. 



1. Plan of Structure. — All vertebrate animals are obliged to go in 

 search of their food, and that for the most part over an extensive range. 

 Consequently they must be endowed with the means of free locomotion. 

 (Contrast them in this respect with plants and many animal parasites.) 



WT R.m. 



Longitudinal Section through the Body of a Vertebrate (Diagrammatic). 



W., Vertebral column ; R.m., spinal marrow ; G., brain ; S.n., optic nerve ; K. n., 

 olfactory nerve, ramifying within the nasal cavities; Z. , tongue; Sp. , oesophagus; 

 Ma., stomach ; D. , intestine ; L. , liver ; B.dr. , pancreas ; Mi., spleen ; Lr., trachea ; Lu., 

 lung ; H., heart, with several bloodvessels ; Z\\\, diaphragm ; TS. , kidney ; H.I., ureter ; 

 H.b. , urinary bladder. 



The smaller the resistance encountered by a creature in its progress 

 through the air, the water, or on the earth, the more rapidly it can move, 

 and the more easily is it able to obtain its food. Now, motion is most 

 easily accomplished in the case of a body of rod-like, cylindrical shape, 



