Chap. 13 



THE RELEASE OF ENERGY RESPIRATION 



229 



Fig. 13.4. Respiratory organs of in- 

 vertebrates (continued). Tracheal gills of 

 the aquatic stage of a mayfly (Epeorus). 

 The tracheae contain gases but ordinarily 

 no fluid. The leaf-like gills extend from 

 the abdomen; their movements are quick- 

 ened whenever oxygen is sparse. The 

 tracheae are visible as a tracery of dark 

 lines upon each gill and in the body. Total 

 length of insect, one inch. 



B 



folds (oral valves) are bent backward allowing the water to pour in (Fig. 

 13.6). The water at once expands the cavity and presses the folds together. 

 The esophagus is contracted so that little or no water is swallowed and for a 

 moment it is also prevented from moving out through the slits at the sides 

 of the mouth (pharynx) by the closure of the opercula (Figs. 13.6, 13.7). 

 The floor of the mouth is then raised, the opercula are lifted, and the water 

 escapes through the gill slits. As it does so it washes the slender filaments of 

 the gills (attached to the gill arches) that contain the circulating blood. This is 

 the moment when the exchange of gases takes place. As soon as the water 

 passes out of the mouth (pharynx), the opercula close and another breathing 

 action begins. Each time that water passes over the gills, food contained in it 

 is caught on the strainers called gill rakers (Fig. 13.7). Fishes do not neces- 

 sarily close their mouths when breathing, but simply open them wider when 

 they inhale water. 



Gills are significant only in connection with the circulation (Fig. 13.6). 

 The two main chambers of the heart of fishes lie below the pharynx. Venous 



