848 



EESPIKATION IN LOWEB ANlMAIjS 



fringes by constantly taking in and forcing out water 

 through slits in the side of the head. The rich supply of 



blood capillaries in 

 these fish gills is in- 

 dicated by their red 

 color. The blood 

 of the fish passes 

 from the ventricle of ■ 

 the heart into these 

 capillaries ; there ex- 

 changes its carbon 

 dioxide for oxygen 

 and then passes di- 

 rectly over the body 

 without first return- 

 ing to the heart, as 

 in our bodies. 



While the gill is well adapted to gas exchange with water 

 the small proportion of oxygen found in water makes that 

 element a poor source of oxygen, and all animals higher 

 than the fishes obtain their supply of this gas from a me- 

 dium much richer in it, namely, air. Such animals extract 

 their oxygen supply from air by structures called lungs. 



Lungs. — Just as we defined a gill as an external fold of 

 the skin, so we may define the lung as an internal fold 

 of the skin lined with capillaries and adapted for the 

 exchange of oxygen and carbon dioxide between the blood 

 and the air. 



The lung of the snail is the simplest type. Here, the 

 single, internal pocket, lined with blood vessels, may be 

 conceived of as having been developed by a single infold 

 of the external skin. So slow is the exchange of gases in 



Fig. 159 — Gills and heart of tlie perch, exposed 

 by removal of gill cover on left side ; a, iirst of 

 the four bony arches which carry the gills ; b, 

 gills ; 6', lower edges of gills on the right side ; 

 kt heart. 



