8o FIRST LESSONS IN ZOOLOGY 



pass through a thin, moist membrane. Thus if a closed 

 sac made of thin membrane filled with water in which 

 carbon dioxide is dissolved be immersed in water in which 

 oxygen is dissolved, carbon dioxide will pass out of the 

 sac and oxygen into it until there is the same amount of 

 each outside and inside. If the water outside is con- 

 stantly replaced all the carbon dioxide will be finally 

 removed. If the o-X)'gen inside the sac is constantly 

 used up and the suppl)' outside is ahva\s renewed, o.xygen 

 will be constantl)' going in and carbon dioxide going 

 out. This is just what happens in the living animal. 

 Animals get their oxygen from the air, of which it is a 

 part. The air may be free or dissolved in water. Carbon 

 dioxide is made in the cells of the body. Respiration 

 takes place through the membranes covering all or part 

 of the surface of the body. It requires the constant re- 

 newal of free air or \\'ater containing air on the outside, 

 and the constant passage of fresh blood on the inside sur- 

 face of the membrane. This end is attained in a variety 

 of ways among animals. 



In tjie simplest forms, the Protozoa, where we have 

 the most [)rimitive means of motion, we find also the 

 simplest means of respiration. The Am(eba (see Chap- 

 ter IX) simply relies on its wliole external surface for 

 breathing, the thin outside layer of the body acting as a 

 membrane through which the o.xygen passes in and the 

 carbon dioxide out. During periods of activity the pro- 

 cesses protruding from the body increase the amount of 

 respiratory surface sufficiently to provide for the increased 

 respiration demanded by the activit)'. In ciliated forms the 

 cilia grcath' increase the surface area and respiration is 

 further assisted by the constant contact of the moving 

 body with fresiier water. ICven in more complex animals, 

 tlie common earthworm and the larv;e of some insects 

 for e.vample, the whole external sl^in is sometimes the 



