ONE-CELLED AND MANY-CELLED ANIMALS 43 



body must be so arranged or composed as to be capable of 

 performing these necessary life processes. All animals take 

 food, digest it and assimilate it, that is, convert it into new body 

 substance; all animals take in oxygen and give off carbon 

 dioxide; all animals have the power of movement or motion 

 (not necessarily locomotion); all animals have the power of 

 sensation, that is, can feel; all animals can reproduce them- 

 selves, that is, produce young. These are the necessary life 

 processes. It is evident that the dog could still live if it had 

 no eyes. Seeing is not one of the necessary functions or proc- 

 esses of life. Nor is hearing, nor is leaping, nor are many of 

 the other things which the dog can do; and animals can exist, 

 and do exist, without any organs to enable them to see and hear 

 and leap. But the body of an animal must be capable of 

 performing the few essential processes which are necessary to 

 animal life. How surprisingly simple such a body can be our 

 study of the Protozoa has already shown. But in most animals 

 the body is a complicated object, and is able to do many things 

 which are accessory to the really essential life processes, and 

 which make its life complex and elaborate. 



The Principal Systems of Organs and Functions. These 

 complex life processes are usually carried on by systems of 

 organs which are known as the skeletal, muscular, digestive, 

 respiratory, circulatory, excretory, nervous and reproductive 

 systems. And the particular set of special functions or life 

 processes connected with each is sufficiently indicated by its 

 name. Of them all, the reproductive system and its function, 

 which is that of the multiplication of the species, calls for a few 

 special words of introduction before we pass to the considera- 

 tion of the successive animal groups. For it is in connection 

 with this function that some of the most important special 

 conditions in animal life exist. The important fact of sex, for 

 example, is correlated with this function, while the whole 

 subject of animal development may be looked on as part of 

 the study of animal multiplication. 



Reproduction and Development in the Metazoa. We have 

 learned that the process of multiplication among the Protozoa 

 is, in most cases, very simple, consisting of the simple splitting 



