XIII 



PHYLUM CHORDATA 



237 



in stagnant water, the oxygen in the bladder, which normally 

 amounts to 20 or 25 per cent., is entirely absorbed and replaced 

 by nitrogen and carbonic acid. Its normal function however, is 

 hydrostatic, i.e. it serves to keep the Fish of the same specific 

 gravity as the water. The specific gravity of the Fish as a whole, 

 falling or rising as it must on account of the increase or decrease 

 of pressure at various depths as the Fish descends or ascends 

 causing greater or lessjcompression of the gases in the air-bladder, 



opt 



crh 



tx..bl 



Fig. 894. — Horizontal section of posterior portion of liead and anterior end of air-bladder in 

 Fseudophycis bachus, one of the Gadidfe or Cods (semi-diagrammatic), a. thickened 

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

 bladder ; aw. cp. outer wall of auditory capsule ; au. cp\ inner (membranous) waU ; b, 

 hollow offshoots of air-bladder ; cj3. str. corpora striata ; crb. cerebellum ; iiUTrib. lab. mem- 

 branous labyrinth ; olf. I. olfactory bulbs ; olf. p. olfactory peduncles (olfactory tracts) ; 

 op. operculum ; opt. I. optic lobes ; vs. gn. vaso-ganglia. 



can be brought to approximate to that of the surrounding water 

 by increase or decrease in the quantity of the contained gas. This 

 is brought about by secretion or absorption, often by means of 

 vaso-ganglia or red glands (Fig. 894, vs. gn). These are elevations 

 of the wall of the bladder, abundantly supplied with blood, and 

 containing tubular glands which open into the cavity of the 

 bladder. In Fishes with a pneumatic duct the red glands 

 are absent, but in Eels their place is taken by red bodies 



