AIR-BLADDEK. 143 



quently prolonged into the tail, the prolongation being either 

 simple and lodged between the non-united parapophyses, or 

 double and penetrating between the muscles and hsemapo- ' 

 physes of each side. In the opposite direction processes of 

 the air-bladder may penetrate into the skull, as has been 

 mentioned above (p. 117). In some fishes the air-bladder is 

 almost loose in the abdominal cavity, whilst in others it ad- 

 heres most intimately by firm and short tissue to the vertebral 

 column, the walls of the abdomen, and the intestines. In the 

 Cobitina and many Siluroids it is more or less completely 

 enclosed in osseous capsules formed by the vertebroe. 



The tunics of the majority of air-bladders are an extremely 

 fine internal one, frequently shining silvery, containing crys- 

 talline corpuscles, sometimes covered with a pavement- 

 epitheKum ; and a thicker outer one of a fibrous texture, 

 which sometimes attains to considerable thickness and yields 

 isinglass. This wall is strengthened in many fishes by mus- 

 cular layers for the compression of the whole organ or of 

 some portion of it. 



A distinction has been made between air-bladders which 

 communicate by a duct with the intestinal tract and those 

 which are entirely closed. However, it is to be remembered 

 that at an early stage of development all air-bladders are 

 provided with such a duct, which in a part of the fishes more 

 or less completely obliterates, being then represented by a fine 

 ligament only. In young Lucioperca of six to eight inches in 

 length the duct may be found still open for a considerable 

 distance ; and, on the other hand, in adult Physostomi, that is 

 Teleosteous fishes with a ductus pneumaticus, not rarely the 

 whole duct is found very narrow, or, for some part of its 

 length, even entirely closed. 



Air-bladders without duct are found in Acanthopterygians, 

 Pharyngognaths, Anacanths, and Lophobranchs. They may 

 consist of a single cavity or divided by constrictions into two 



