HYDROSTATIC AND ACOUSTIC USES OF. ' xliii 



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

 returned from it ; whereas .^in the higher forma venoria 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 {FhysocUsti), as in the spiny-rayed Acanthojpterygians, the spineless 

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

 nathi. While in the remaining orders a connecting duct remains 

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



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 8. scomber 

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



In such forms as swim neat* the surface the air-bladder ia generally of a 

 compai-atively small size ; while in those which live near the bottom, as the 

 flat fishes, Pleuronedidce, 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 hooked 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.shes mouth.* 



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

 sound) in 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 varicius 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 Percidse, wherein its greatest length is in .its longitudinal axis. In 

 some forms, as " Holocentrum and Sargus coecal processes of the air-bladder 

 diverge to attach themselves to the meuibrane, 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 

 column of water on the air contained in the. bladder." — Muiler. 



