XIII 



PHYLUM CHORDATA 



221 



function, however, is hydrostatic, i.e. it serves to keep the Fish of the 

 same specific gravity as the water. Variations in pressure as the 

 Fish ascends or descends are regulated by absorption or secretion of 

 gas, often by means of vase-ganglia or red glands (Fig. 844, cs. gn. ) 

 in the walls. These are elevations of the wall of the bladder, 

 abundantly supplied with blood, and having tubular glands which 

 open into the cavity of the bladder and secrete a fluid of unknown 

 function. In Fishes with a pneumatic duct the red jjlands an- 



opt. I 



FIG. 844. Horizontal section of posterior portion of head and anterior end of air-bladder in 

 Pseudophycis bachus. one of the Gadidse or Cods (semi-diagrammatic), a, thickened 

 portion of air-bladder fitting into fenestra in posterior wall of auditory capsule ; a. bl. air- 

 bladder ; au. cp. outer wall of auditory capsule; an. cp.' inner (membranous) wall; l>. 

 hollow offshoots of air-bladder ; cp. str. corpora striata ; crb. cerebellum ; mcmb. lab. mem- 

 branous labyrinth ; olf. 1. olfactory lobes ; olj. p. olfactory peduncles ; op. operculum ; opt. I. 

 optic lobes ; cs. gn. vaso-ganglia. 



absent, but in Eels their place is taken by red bodies of similar appear- 

 ance but with non-glandular epithelium. In some forms with closed 

 air-bladder the anterior end of the organ is forked, and each branch 

 (a) fits closely against a membranous space in the posterior wall of 

 the auditory capsule, while laterally it extends outwards in the 

 region of the shoulder-girdle, and comes to lie immediately beneath 

 the skin ; in this way varying pressures on the surface of the body 

 are transmitted through the air in the bladder to the auditory 



