OUTLINES OF ZOOLOGY. 



CHAPTER I. 



GENERAL SURVEY OF THE ANIMAL KINGDOM. 



The simplest animals (Protozoa) are mostly unit masses 

 of living matter or single cells, and are thus contrasted with 

 all the other animals which are many-celled (Metazoa), 

 But these rigid divisions are never quite true, for some 

 Protozoa form balls of cells and thus bridge the gulf. 



It is customary to divide the Metazoa into two sets, back- 

 boneless animals (Invertebrata) and backboned animals 

 (Vertebrata), and the distinction is practically useful. 

 Nowadays, the favourite terms are Non-Chordata and 

 Chordata. We may contrast the two sets as follows : — 



Backboneless, Invertebrate, 

 Non-Chordate. 



Backboned, Vertebrate, 

 Chordate. 



Ventral, or lateral, never dorsal nerve- 

 cords. 



The eye is usually derived directly from 

 the skin. 



The heart is dorsal. 



Dorsal nerve-cord (spinal cord, brain). 



Dorsal supporting rod (notochord, re- 

 placed in most by a backbone). 



Gill-slits (or visceral clefts) from the 

 pharynx to the exterior (not used for 

 respiration above amphibians). 



The essential parts of the eye are 

 formed from an outgrowth of the 

 brain. 



The heart is ventral. 



