xlvi AIR-BLADDEE. 



hydrostatic functions, and as a general rale being smaller in fresh-water 

 than in marine species. The Siluridce of Asia live the life of ground-feeders, 

 and the power of employing their air-bladder as a float appears to be 

 subservient to that of hearing. In the marine forms it has thickened walls, 

 and the parapophyses of the first vertebras (ex. Arms subrostratus) form 

 expanded plates, to the under surface of which this organ is attached. As 

 we go inland, especially towards the Himalayas, this organ becomes more, 

 and more enveloped in bone until it is as we find it in the loaches, while, 

 like the Cyprinidas, a chain of ossicles passes' forwards to the internal ear. 

 In the marine forms the broad plate on its upper surface and bony stays 

 to its partitions would appear to exist for the purpose of counteracting 

 superincumbent pressure ; while in the fresh-water forms this bony covering, 

 being greatly increased, would seem to be due to some fresh-water physical 

 cause, not' to a tropical climate, as I have observed the same phenomena 

 is seen in European loaches. As all are ground-feeders, one reason at 

 least must be to' prevent undue pressure on that- organ when at great 

 depths, and ..to preclude any abnormal interference with the function of ' 

 hearing. 



Among the marine Physostomous forms all that have been examined, 

 I believe, and in which communications exist between the air-bladder and 

 internal ear, have such by means of coecal prolongations from the air-bladder, 

 and not by a chain of auditory ossicles, which appear to be absent in marine 

 fishes. This leads one to inquire whether there are any fresh- water fishes 

 that have this connection, as observed in sea forms. The perch has no 

 auditory ossicles, ancTl should think. its origin may be given as marine; 

 and. the same conclusion may be come to of the trout and anadromous 

 salmon. 



In short, the air-bladder in fishes is the homologue of the lungs of the 

 superior vertebrate forms ; in some of the higher sub-classes it serves as a 

 lung, depurating the blood ; but in the majority of true or teleostean fishes 

 it is employed for one or both of the -following purposes : as a float, enabling 

 its possessor, by compressing or dilating it, to sink or rise to any desired 

 level in the water ; or, secondly, that it assists hearing by communicating 

 with the internal ear : that in those forms in which it has ah auditory 

 function, we perceive two very distinct modifications, for among marine 

 Physoclisti a prolongation of the air-bladder, passes forwards to the interior 

 of the skull, while in fresh- water Physostomi, although a similar connection 

 exists, it is by means of a chain of auditory ossicles y while in such fishes 

 as live the life of ground-feeders a still further change may take place, in 

 the air-bladder itself being more or less completely surrounded by osseous 

 walls, formed by the growth of the parapophyses of some of the anterior 

 vertebras. . . 



