86 ^RESPIRATION OF FISHES. 



fishes, these lamella? are simple, and only attached at their base ; 

 in a small number, they are, on the contrary, ramified, and in the 

 form of tufts ; finally, in most cartilaginous fishes, they are 

 attached to the skin by their external edge, as well as to the 

 arches of the branchiae by their internal edge. 



31. The water necessary for respiration enters the mouth, and, 

 by an act of swallowing, passes through the slits, left between the 

 branchial curves or arches, and in this way reaches the branchiae, 

 laves their surfaces, and then escapes through the openings of the 

 gills. We see, in fact, the animal open its mouth, and elevate the 

 operculum alternately. In fishes, in which the branchiae are 

 free on their external edge, one of these openings on each side 

 is sufficient ; but, when the branchiae are fixed, there is required 

 as many openings as there are spaces betwixt the branchiae. 

 Consequently, we are made acquainted with the arrangement of 

 the respiratory apparatus by simply inspecting the external 

 openings. 



32. Fishes consume a very moderate quantity of oxygen, some 

 however, are not content with what is dissolved in the water, 

 and visit the surface, from time to time, to breathe the air. There 

 are some indeed that swallow it, and by causing it to pass 

 through the intestine, convert the oxygen into carbonic acid. 

 When fishes remain out of water, they generally perish very 

 quickly from asphyxia, not for the want of oxygen, but because, 

 the branchial lamellae, being unsustained by the water, are effaced, 

 and do not permit the blood to pass readily through them, and 

 because these organs, by drying, become unfitted for performing 

 their functions : therefore, those fishes that perish most promptly 

 from exposure to the air, have widely open gills, which facili- 

 tates evaporation from the branchiae, while, those that resist this 

 exposure best, have these openings ver}' narrow, or even possess 

 some receptacle in which they preserve water for moistening 

 these organs. 



33. As we have already stated, fishes produce scarcely any 

 hrat: but some of them possess the singular faculty of producing 

 electricity, and of giving very powerful shocks to animals that 

 touch them. The Torpedo, the Silurus, and a species of Gym- 

 notus. are of this kind, and what is very remarkable, the confor- 

 mation of the electric organ differs in each one of them. 



31. How do the gills receive the supply of water necessary for the respira- 

 tion of tithes ? 



32. Do fishes require a large quantity of oxygen for the purposes of res- 

 piration ? Why do fishes quickly die when out of the water? 



33. Do fishes p-odnce much animal heat ? Do all fishes possess the faculty 

 of producing electricity? 



