128 ANIMALS 



a structure and function not duplicated by any other cell in the 

 body. The gap between the simplest ^nd most complex 

 animals is occupied by many types of intermediate degrees of 

 complexity and we shall therefore keep in mind several of the 

 most significant of these types while seeking to obtain a concep- 

 tion of what constitutes an animal. 



303. Animal Types. — As an example of the very simplest 

 kinds of animals we shall frequently refer to the amoeba, a 

 minute speck of living jelly, quite common in the bottom slime 

 of ponds. 



304. As a sUghtly more complex form we will take hydra, 

 which is also found in fresh-water ponds, attached to plants 

 or other objects in the water. It is vase-like in form and has a 

 circle of long slender arms or tentacles near the oral end. 



305. As a still more complex form the common earth- 

 worm may serve very well, or a segmented marine worm, like 

 nereis. 



306. The crayfish will form another step forward. This 

 animal is also common in most parts of the world and should 

 be familiar to everyone. In this connection reference will 

 occasionally be made to insects, which belong to the same 

 phylum. 



307. As an example of the most complicated type of 

 animal organization any mammal may be kept in mind, such 

 as the cat, dog or rabbit, or, better still, man. The student 

 will be supposed to have some knowledge of human anatomy 

 and physiology. Reference will also be made to other members 

 of the vertebrate phylum, such as fishes, frogs, reptiles and birds. 



308. Color and Form. — If we compare plants and animals 

 with regard to color and form we find nothing in common. 

 Animals contain no chlorophyl and they are therefore physio- 

 logically dependent upon plants. Nor do they have any other 

 general color characteristic. The form of plants we found was 

 determined by the necessity of exposing chlorophyll tissue to 



