230 



ZOOLOGY 



SECT. 



causing greater or less compression of the gases in the air-bladder, 

 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. 905, vs. gri). 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 



opt. I 



FlO. 905. Horizontal section of posterior portion of head and anterior end of air-bladder in 

 Fseudophyeis bachus, one of the Gadidae 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 ; au. cp'. inner (membranous) wall ; b. 

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

 branous labyrinth ; oJf. 1. olfactory bulbs ; olf. p. olfactory peduncles (olfactory tracts) ; 

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



in the Eels and others their place is taken by red bodies of similar 

 appearance, but with non-glandular epithelium. In some forms 

 with closed air-bladder the anterior end of the organ is forked, 

 and each branch (Fig. 905, 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 organ. In the Carps and Siluroids a 



