A STUDY OF RESPIRATION 229 



5. A COMPARATIVE STUDY OF EESPIBATION 



Respiration in Single-celled Animals. Amoeba, parame- 

 cium, and other single-celled animals take in oxygen all over 

 the surface of the body, obtaining their supply from the air 

 that is held in suspension by the water. In these animals 

 of small size the oxygen can easily penetrate to all portions 

 of the protoplasm, and hence no circulatory system is neces- 

 sary for its distribution. Carbon dioxid can likewise be sent 

 off from all parts of the cell surface. 



Respiration in the Earthworm. Eespiration in the earth- 

 worm is carried on through the skin. Any one at all familiar 

 with the habits of these animals knows that their skin must 

 be kept moist, otherwise they die. The capillary blood ves- 

 sels pass close to the surface in order to supply the blood 

 with oxygen and to excrete the wastes. If the skin becomes 

 dry, the blood loses a great deal of water by evaporation, 

 and the hardened outer surface shuts off the supply of 

 oxygen. 



Respiration in Fishes. The water in which fishes live is 

 composed of one part oxygen and two parts hydrogen (H 2 0). 

 Animals, however, are unable to obtain free oxygen by 

 separating it from the hydrogen, and the oxygen they use 

 is supplied by the air dissolved in the water. At the sides 

 of the mouth cavity of a fish are slitlike openings. The 

 water taken in by the mouth is forced through these open- 

 ings, over the four or five pairs of comb-shaped gills, to the 

 outside of the body. The single ventricle of the fish heart 

 forces the blood out to the gills, where the arteries connect 

 with a great number of capillaries running close to the gill 

 surface. Here the blood takes up a supply of oxygen and 

 loses many of its waste matters (see Fig. 60). 



Respiration in Air-breathing Animals. Toads and frogs, 

 when hatched from the egg, begin their life as tadpoles. In 

 this state they are really fishes. They breathe by gills ; and 

 the blood circulation is like that of a fish. While the legs 



