142 



15. Fish mtliout air-vessels arc usually such as frequent the bottom of 

 the water ; but there are exceptions to this rule — in some an increased muscular 

 development seems to render it unnecessary, and in others there is no clear 

 explanation of its absence (applause). 



(The paper was illustrated by dried specimens of air-bladders and outline 

 Bketchea.) 



The Rev, W. HoDGHTON said he could make many remarks on Mr. Lloyd's 

 interesting paper, but he should confine what he had to say to a few points, as 

 there was very little time for discussion. As to the gaseous contents of the air- 

 bladders of fish, he confessed he shared with the late Dr. Davy considerable 

 doubt. That the same organ should secrete two such different gases as nitrogen 

 and oxygen seemed certainly very anomalous. It was generally believed that 

 salt-water fishes secreted oxygen, and fresh-water fishes nitrogen, in their air- 

 bladders, Hiunboldt, experimenting on the Gymnotus Electricus, of South 

 America, found the gas to consist of 96 per cent, of nitrogen and 4 per cent, of 

 oxygen. M. Biot, on the other hand, experimenting on some deep-sea fishes of 

 the Mediterranean, found 87 per cent, of oxygen, and the rest nitrogen, with » 

 trace of carbonic acid. The secretion of oxygen by any animal was remarkable, 

 and one might as well expect this gas to be exhaled from the lungs in respira- 

 tion, as separated from the blood by secretion from the inner t\inic of the swim,- 

 bladder. Mr. Houghton did not mean to deny the results obtained by 

 Humboldt and Biot, but he thought the matter required further verification. 



As to the function of the swim bladder he regarded it as simply a 

 mechanical one as affecting the specific gravity of the fish ; but he did not 

 agiee with Mr, Lloyd in regarding its presence as an important organ. This 

 was evident from the fact that in closely aUied species of fish, with precisely 

 similar habits, one species had a swim bladder and another had not. For instance, 

 the common Mackerel has no swim bladder, — one or two other species of 

 Mackerel have one. Of the two British species of sun-fish (Orthrar/oriscus), one 

 has a swim bladder, and the other has not. Some of the SUuroids were pos- 

 sessed of a very complex swim bladder, others again had none at all. Other 

 instances might be given. 



"With respect to the question of the swim bladder of fishes being homo- 

 logous with the lungs of air-breathing vertebrata, this was the most interesting 

 and important point of all. Although, functionally, scarcely a single fish, 

 perhaps, uses the swim bladder as a respiratory organ, yet it was qiiite clear 

 fi-om the case of the mud-fish, or Lepidosircn, that those anatomists who regarded 

 the swim bladder as the homologiie of the lungs, and the i^neumatic duct, 

 where it existed, as the homologue of the trachea of air-breathing vertebrata 

 were correct. The Lepidosiren, whether of the African or South American, 

 rivers, appears to be at one time a Fish, at another a Batrachian, so far at least 

 as its respiration is concerned. Whilst it inhabits the water it breathes by 



