THE SINGLE-CELLED AWMAL BODY 37 



soft speck of viscous matter called protoplasm for a body, 

 the simplest structural condition to be found among living 

 beings, Amoeba nevertheless is capable of performing, in 

 the simplest way in which they may be performed, those 

 processes which are essential to animal life. 



Paramcecium has a body a little less simple than 

 Amoeba. The food-particles are taken into the body 

 always at a certain spot; this might be spoken of as a 

 mouth. And the body has some special locomotory 

 organs, if they may be so called, in the presence of the 

 cilia. The body, too, has a definite shape or form. 

 But, as in Amoeba, there is no alimentary canal, nor 

 nervous system, nor respiratory system, nor reproductive 

 system. The whole body feels and breathes and takes 

 part in reproduction. 



A long jump has been made from the toad and crayfish 

 to Amoeba and Paramoecium; from the complex to the 

 simplest animals. But, as will later be seen, the great 

 difference between the bodies of these simplest animals 

 and those of the highly complex ones is only a difference 

 of degree ; there are animals of all grades and stages of 

 structural condition connecting the simplest with the most 

 complex. When animals are studied systematically, as 

 it is called, we begin with the simplest and proceed from 

 them to the slightly complex, from these to the more 

 complex, and finally to the most complex. There are 

 hundreds of thousands of different kinds of animals, and 

 they represent all the degrees of complexity which lie 

 between the extremes we have so far studied. 



The cell. The characteristic thing about the body of 

 Amccba and Paramcecium and the other "simplest 

 animals ' ' for there are many members of the group of 

 "simplest animals," or Protozoa is that it is com- 

 posed, for the animal's whole lifetime, of a single cell. 

 A cell is the structural unit of the animal body. As 



