ZOOLOGY. 



INTRODUCTION. 



Deflnition of Zoology. — That science which treats of liv' 

 ing beings is called Biology {^ioi, life ; Xoyoi, discourse). 

 It is divided into Botany, which relates to plants, and Zo' 

 ology {^<aov, animal ; Xoyoi, discourse), the science treating 

 of aaimals. 



It is difficult to define what an animal is as distinguished 

 from a plant, when we consider the simplest forms of either 

 kingdom, for it is impossible to draw hard and fast lines in 

 nature. In defining the limits between the animal and 

 vegetable kingdoms, our ordinary conception of what a 

 plant or an animal is will be of little use in dealing with 

 the lowest forms of either kingdom. A horse, fish, or 

 worm differs from an elm tree, a lily, or a fern in having 

 organs of sight, of hearing, of smell, of locomotion, and 

 special organs of digestion, circulation, and respiration, but 

 these plants also take in and absorb food, have a circulation 

 of sap, respire through their leaves, and some plants are me- 

 chanically sensitive, while others are endowed with motion 

 — certain low plants such as diatoms, etc., having this 

 power. In plants, the assimilation of food goes on all over 

 the organism, the transfer of the sap is not confined to any 

 one portion or set of organs as such. It is always easy to 

 distinguish one of the higher plants from one of the higher 

 animals. But when we descend to animals like the sea-ane- 

 mones and coral-polyps which were called Zoophytes from 

 their general resemblance to flowers, so striking is the exter- 

 nal similarity between the two kinds of organisms that the 



