136 



FISHES. 



brought into pure water of normal condition, are obliged to 

 rise to the surface at frequent intervals to take in a quantity 

 of air, and if they be kept beneath the surface by means of 

 a gauze net, they perish from suffocation. The special con- 

 trivances consist of additional respiratory organs, lodged in 

 cavities either adjoining the gill-cavity or communicating 

 with the ventral side of the oesophagus, or of the air-bladder 

 which enters upon respiratory functions (Dipnoi, Lepidosteus, 

 Amia). 



The water used by fishes for respiration is received by the 

 mouth, and by an action similar to that of swallowing driven 

 to the gills, and expelled by the gill-openings, of which there 

 may be one or several on each side beliind the head ; rarely 

 one only in the median line of the ventral surface. 



The gills or hranchice consist essentially of folds of the 



Fig. 58. — Fore-part of the body of au embryon of Carcliarias, showing the 

 brancliial filaments (natural size). 



mucous membrane of the gill-cavity {lamiiuc hranchiales), in 

 which the capillary vessels are distributed. In all fishes the 



