CLASS PISCES 



451 



or lymph. The thyroid is homologous to the endostyle of tuni- 

 cates (p. 391) and Amphioxus (p. 396). It lies in the branchial 

 region and is paired. The thymus is situated dorsally in the 

 branchial region. The .spleen is a large gland usually lying near 

 the stomach ; colorless blood corpuscles are formed in it, and old 

 red corpuscles are destroyed by^it. The suprarenal bodies are 

 situated close to the kidneys. 



The Respiratory System. Respiration takes place in the 

 gills, and, in the DIPNOI and some teleosts, also to some extent 



FIG. 379. Diagram illustrating the mechanism of respiration in teleosts. 

 A, phase of inspiration. B, phase of expiration. (From Wiedersheim, after 

 Dahlgren.) 



in the air-bladder. During respiration in teleosts the walls of the 

 mouth act like a pump (Fig. 379). In inspiration (A) the oral 

 cavity (cav.oris) is enlarged by the raising of the opercular 

 apparatus, and water is therefore drawn into it through the 

 mouth. Folds of mucous membrane (branchiostegal mem- 



