278 LECTURE XI. 



which suffices for all the evolutions they are called upon to per- 

 form. With regard to the accessory offices of the air-bladder in 

 relation to the sense of hearing, the chief of these remarkable modifi- 

 cations by which it is brought into communication with the acoustic 

 labyrinth have been already described in a former Lecture (p. 210.). 

 In a few genera {Trigla, Pogo7iias) the air-bladder and its duct 

 are subservient to the production of sounds. 



Under all its diversities of structure and function the homology of 

 the swim-bladder with the lungs is clearly traceable ; and finally, in 

 those orders of fishes which lead more directly to the Reptilia, as, for 

 example, the salamandroid Ganoidei and Protopteri, those further 

 modifications are superinduced upon the air-bladder, by which it 

 becomes also analogous- in function to the lungs of the air-breathing 

 Amphibia. 



The species of Lepidosiren, the anatomy of which is described in 

 the Linntean Transactions * and in these Lectures, inhabits a part of 

 the river Gambia, which in the rainy season overflows extensive 

 tracts, that are again left dry in the dry season. The Lepidosirens, 

 which do not follow the retreating waters, escape from the scorching 

 rays of the African sun by burrowing in the mud, which is soon 

 baked hard above them ; but they maintain a communication with 

 the air by a small aperture, and coiling themselves up in their cool 

 chamber, clothe themselves by a layer of thick mucous secretion, and 

 await, in a torpid state, the return of the rains and the overflowing 

 of the mud-banks. The advent of their proper element wakes them 

 into activity : they then emerge from the softened mud, swim briskly 

 about, feed voraciously, and propagate. 



The peculiar modifications of the gills and air-bladder of the Lepi- 

 dosiren are precisely those which adapt them to the peculiar con- 

 ditions of their existence. Li the inactive state into which they are 

 thrown by their false position as terrestrial animals, the circulation, 

 which would have been liable to be stopped had all the branchial 

 arteries developed gills, as in normal fishes, is carried on through 

 the two persistent primitive vascular channels {fig- 71. 2 and 3), 

 Whatever amount of respiration was requisite to maintain life during 

 the dry months is efiected in the pulmonary air-bladders ; its short 

 and wide duct or trachea, the oesophageal orifice of which is kept 

 open by a laryngeal cartilage, introduces the air directly into the 

 bladders : the blood transmitted through the bi'anchial arches to the 

 pulmonary arteries (ib. V) is distributed by their ramifications over 

 the cellular surface of the air-bladders, and is returned arterialised 



