HYDROSTATIC AND ACOUSTIC USES OP. xliii 



sanguification,, in the lower division arterial blood goes to it and venous is 

 returned from it; whereas in the higher forms venous is carried to it, 

 oxygenated at it, and returned as arterial blood into the heart. 



In teleostean fishes the air-bladder exists in the form of a closed 

 sac (Physoclisti) , as in the spiny-rayed Acantliopterygians, the spineless 

 Anacanthini, the tufted-gilled Lophobranchii, and the hard-jawed Plectog- 

 nathi. While in the remaining orders a connecting duct remains 

 pervious, as in the Physostomi, excluding the family Scombresocidce. 



The air-bladder, however, is not only absent in many families, but it 

 may be present or deficient among species of the same genus. One form of 

 British mackerel, Scomber colias, possesses this organ, while the S. scomber 

 has none. This is by no means peculiar to European genera. 



In such forms as swim near the surface the air-bladder is generally of a 

 comparatively small size ; while in those which live near the bottom, as the 

 flat fishes, Pleuronectidce, it is as a rule absent. In species possessing this 

 organ, should it become ruptured from any cause, permitting the contained 

 gas to escape, the fish has by some authors been observed to sink to the bottom, 

 and to be unable to re-ascend, a conclusion some experiments have failed 

 to establish. On the other hand some forms which have been hooded or 

 netted at great depths and suddenly brought to the surface, without having 

 time to- compress or partially empty their' air-bladders, the contained gas 

 being no longer weighted down by. a mass of superimposed water, expands 

 rapidly, causing the organ to burst, or else forcing the stomach and upper 

 portion of the alimentary canal into the fi.sh.es mouth.* 



The chief use of this' organ (excluding respiration and the production of 

 sound) iu teleostean fishes are two' — (1) A hydrostatic, or for flotation, 

 which serves by contracting or distending its capacity, to condense or 

 rarify the contained gases, giving it the mechanical function of enabling its 

 possessor to maintain a desired level in the water, and which is accompanied 

 with the power of renewing, expelling, and compressing, or dilating its gaseous- 

 contents, so that it can rise or fall as necessity occurs. (2) The second 

 use is acoustic, it being partially or entirely employed for hearing, by 

 means of various modes of connection with the internal ear, mostly by- 

 tubular prolongations of the air-bladder, or a connecting chain of auditory 

 ossicles. 



Among the Physoclisti, the majority of which are marine, we find the 

 air-bladder as a closed sac, having a single cavity, as observed in many of 

 the Percidaj, wherein its greatest length is in its longitudinal .axis. In 

 some forms, as " Hvlocentrum and Sargus ccecal processes of the air-bladder 

 diverge to attach . themselves to the membrane, closing the part of the 



• " A fish may remain at the bottom of the water due to the very fact of the pressure of the 

 colnmu of water on the air contained in the bladder."— Mulleb. 



