RESPIRATION OP FISHES. 17 



have an opening for every one of the spaces between the gills. 

 Thus, in the Shark we find five, and in the Lamprey (Fig. 285) 

 seven pairs. We can therefore know the arrangement of the 

 respiratory apparatus, by the single inspection of its external 

 openings. It is also observed that amongst some fishes, the 

 water does not pass directly from the mouth into the respiratory 

 cavity by the openings situated between the branchial arches, 

 but arrives there by a canal situated beneath the oesophagus, 

 something like the trachea of the higher animals ; the Lampreys 

 show this kind of structure. 



538. Fish consume but a small quantity of oxygen ; some, 

 however, are not satisfied with that which is dissolved in the 

 water, and come to the surface from time to time to breathe 

 air. There are even some which make use of it by swallowing it, 

 and converting its oxygen into carbonic acid as it passes along 

 the intestine ; the Loach of our ponds shows us an example 

 of this singular phenomenon. When fish remain out of the 

 water, they die in general very quickly from asphyxia; not 

 because they want oxygen ; but because the branchial plates, 

 being no longer supported by the water, fall together, and thus 

 cannot so easily be traversed by the blood ; and because these 

 organs, when dried up, become unfitted to perform their func- 

 tions. Thus, the fish that perish the most rapidly by exposure 



to the air have their gill- 

 openings very wide, which 

 facilitate the evaporation 

 at the surface of the gills ; 

 whilst those which resist it 

 the best, have very small 

 apertures, or else possess 

 some receptacle, where 

 they can preserve sufficient 

 water to moisten these 

 organs. The different fish 



FIG. 260. RESPIRATORY APPARATUS OF ANABAS. o 



which compose the family 



of the Labyrinthiform Pharyngeans, are very remarkable in 

 this respect, and owe their name to the cellular reservoirs of 



