290 FIRST YEAR COURSE IN GENERAL SCIENCE 



For the most part we are ignorant of these simple organ- 

 isms, because we can neither see them nor taste them. They 

 exist even in drinking water, on the surface of fruits, in the 

 air we breathe. Some are plants and some are animals, and 

 these two classes sometimes have so close a resemblance 

 that it is difficult for those who have not studied them 

 very minutely to determine " which is which." 



The one-celled animal in its general daily life performs all 

 the functions which the higher and more complex animals 

 perform. That is, the one cell carries on the many processes 

 which in the higher animal are divided among various organs. 



As we go somewhat higher in the scale, we find such ani- 

 mals as worms, oysters, snails, lobsters, and insects, which 

 are more complex. They have special organs adapted to the 

 functions of motion, nutrition, and reproduction. 



The highest group of all, the vertebrates, or back-boned 

 animals, is the one to which fishes, reptiles, birds, and the 

 familiar domestic animals belong. Man is the highest type 

 of the vertebrates. 



332. The Nervous System. In the simplest animals, 

 irritability does not belong to any particular part of the 

 body, but is possessed equally by all the protoplasm. But in 

 the higher, many-celled animals, only special groups of cells 

 respond to external conditions. These groups are set apart 

 to take charge of the relation of the animal to its surroundings, 

 and of the relation of one part of the animal to other parts. 

 They perform no other work. Some groups of cells direct 

 the organs of motion; some groups control the work of nu- 

 trition; others are concerned with sensation and, in the 

 higher animals, with thinking, remembering, willing, and the 

 like. All these groups taken together constitute the nervous 

 system. 



333. The Divisions of the Nervous System. The 

 nervous system consists of three divisions: the sense 

 organs, the nerves, and the nerve centers. Each of these 



