346 



KESPIEATION IX LOWER ANIMALS 



pendent of the blood system, and in these animals the blood 

 has practically no part in the exchange of gases. 



Gills. — tUI the higher animals, including fishes, frogs, 

 reptiles, and mammals, breathe by the aid of gills or limgs 

 according as they are water breathers or air breathers. 



A gill in its simplest 

 form is a fold of skin, 

 usually lined with capil- 

 laries and so placed that 

 the oxygen - supplying 

 water shall pass over it, 

 while the thinness of the 

 skin and capillary walls 

 permits the exchange of 

 oxygen and carbon di- 

 oxide between the blood 

 and the water. The 

 pouchlike extensions of 

 the starfish membrane 

 difiei"s from this defini- 

 tion only in the lack of 

 blood capillaries. In 

 this case the exchange 

 of gases is between the 

 body cavity and the 

 water. The blood tubes 

 appear first in the earth- 

 worm where the whole 

 skin acts as a gill. 

 In the clam we find the simple fold relation, exactly as 

 in our definition, and the clam gill may, therefore, be 

 taken as typical. 



Fig. 157 — ^Tlie respiration system of an insect ; 

 a, an air sac ; 6, a traclieal braueli ; c, a spira- 

 cle or breatliing pore. 



