XI 



SENSES AND NERVES 



All animals have a skin of some sort that keeps the things 

 outside their bodies separate from the things inside — that 

 divides the external environment from the internal environ- 

 ment. The sense organs and nervous system keep the two in 

 touch so that the animal can respond to changes in the 

 external environment, a response that is one of the chief 

 signs of being alive, for if an animal no longer responds to 

 such changes it is generally dead. 



Every animal has to keep alive by finding and eating food, 

 and by avoiding being eaten by other animals; secondly it 

 must reproduce its kind so that further generations shall take 

 its place when it dies, for with few exceptions no animals are 

 potentially immortal. The sense organs play an essential 

 part in co-ordinating these fundamental life processes 

 successfully. 



There are some extremely small animals which, although 

 they have no visible sense organs, find their food, breed, and 

 sometimes avoid their enemies. Such animals consist of a 

 single speck of living jelly, similar to that which forms the 

 cells of all the other animals, including ourselves, in which 

 the body is built of millions of cells, each a minute unit 

 separated from its neighbours by a very thin membrane. 



The largest giants among the animals that are not made 

 of a mass of cells stuck together (the Protozoa) are no larger 

 than a pin-head ; most of the others can be seen only by the 

 aid of a microscope. The Amoeba is a well-known example 

 of the Protozoa which can be found everywhere in stagnant 

 water, where it crawls about on the bottom or on the surface 

 of submerged objects. It has no definite shape and its outline 

 continually changes as it moves about and pushes out irregu- 

 lar lobes from different parts of its surface. The animal 

 crawls by pushing out a lobe ahead into which the rest of 



