RESPIRATORY, HYDROSTATIC, AND ACOUSTIC USES OF. xli 



with the alimentary canal, into which latter it opens usually on its superior 

 or dorsal, rarely on its lateral, but occasionally on its inferior or ventral wall. 



The air-bladder is present in the sub-classes Dipnoids, all the members 

 of which are fresh-water; also in the Ganoids, which are entirely or partially 

 fresh-water fishes. It is absent in the Chondropterygii, except in a rudi- 

 mentary form, as a diverticulum opening on the dorsal wall of the pharynx, 

 and then only in some genera. While in the lowly developed Cyclostomata 

 (Mar.npobranchii) and the Leptocardii (Pharyngobrancliii) it is entirely 

 wanting. 



In the large sub-class Teleostei it is present in the Orders Acanthopterygii, 

 Lopliobranchii, and Plectognathi as a closed sac (PJtysoclisti), the majority of 

 these fishes are marine. Whereas in the Physostomi it generally exists as a 

 sac, having a connecting tube opening from it into the alimentary canal ; 

 the largest proportion of which fishes live in fresh water. As a general rule 

 the air-bladder exists more constantly in the fresh-water than in the marine 

 classes. 



Among the Dipnoids it is distinctly or indistinctly double, lung-like, and 

 communicating throughout life, by means of a duct and glottis, with the 

 oesophagus or pharynx. It is thus in the Lepidosiren of. the Brazils, in 

 South America ; Protopterus of tropical Africa; and Geratodus of Queens- 

 land, South Australia. All these fishes have been observed to ascend to 

 the surface of the water for the purposes of respiration, and take in atmos- 

 heric air direct ; a mode of breathing, however, not confined solely to these 

 forms.* 



The Acipenser or sturgeon is provided with an air-bladder, but not 

 similarly cellular to those already remarked upon ; it does not appear to be 

 employed in respiration, but its functions seem to be restricted to acting as- 

 a float. The opening from the air-bladder into the alimentary canal is that 

 of a lower type, being on the dorsal surface, and it is not guarded by a glottis. 



Thus among the Dipnoids.the air-bladder has a lung-like function, opening 

 on the ventral wall of the pharynx, and protected by a glottis. But among 

 the Ganoids there is a divergence from the Dipnoid organization. Although 

 Amia has a respiratory air-sac, opening into the pharynx by a glottis, the 

 communicating orifice is on the upper or dorsal, and not on the lower or 

 ventral aspect, as occurring in the higher forms. Polypterus it is true opens 

 on the ventral wall, but in Acipenser the respiratory function, has not been 

 developed, and the pneumatic opening is upon the dorsal wall of the 

 pharynx. 



Air-bladders in fishes as might be. expected in organs appearing in the 

 lowest class of vertebrates, are of diverse forms and sizes, as well as 

 differently protected. Existing in the abdominal cavity, as already described, 



* See Day, Cotteswold Naturalists' Field Chub, vol. vi, pp. 229-242. 



