450 ANIMAL RESPIRATION 



might possess some interesting modifications of the swim-bladder 

 to enable them to breathe the air. It was not, however, until . . . 

 1887 that an explanation of the mystery of their respiration 

 occurred to me namely, that the respiration is mainly performed 

 by the tail. Since then Professor H addon has been carrying on 

 some experiments in Torres Straits, and has shown that this ex- 

 planation is correct. It seems at first sight a very extraordinary 

 thing that a fish should have become so modified by change of 

 habit as actually to have transferred the chief part of its respira- 

 tory functions from its gills to its tail. It is a well-known and 

 generally recognized fact, however, that in all the Amphibia the 

 skin plays a very important part as an organ of respiration, and 

 it is quite possible that the thin skin between the fin -rays of many 

 fishes also acts as an accessory to the gills and performs the same 

 function. If this is proved to be the case we should have to look 

 upon the tail of Periophthalmus as an example of an organ dis- 

 charging a function which is performed in a lesser degree by the 

 tails of many if not of all fishes." 



Among freshwater forms some of the carp -like fishes, such 

 as the Bleak (Alburnus lucidus) and Roach \Leuciscus rutilus] 

 (fig. 562), are definitely known to rise from time to time to the 

 surface in order to swallow air, since the oxygen dissolved in the 

 surrounding water is not sufficient for their breathing processes. 

 It therefore follows that such forms, extraordinary though the 

 statement may seem, can be drowned by keeping them below 

 the surface. Definite experiments have been made which prove 

 this. For example, Semper (in Animal Life] makes the follow- 

 ing statement: "If we prevent the species of Leuciscus [i.e. 

 Roaches, &c.] from coming to the surface of an aquarium by 

 placing a wire net just below the surface of the water, so that 

 they cannot gulp the air, they soon die, even when an ample 

 supply of highly aerated water is constantly added ". It is sug- 

 gested that part of this air is breathed by the lining of the 

 digestive tube, and this is definitely known to be the case in the 

 small freshwater fishes called Loaches, which also are in the 

 habit of swallowing air. Two species of Loach are native to 

 Britain, the Common Loach (Nemachilus barbatulus) and the 

 Spiny Loach (Cobitis tencza). But all these freshwater forms 

 quickly die when taken out of the water. 



It is, however, by tropical freshwater fishes that a double 



