AIR-BREATHING IN FISHES 33 



(c) THE SWIMBLADDER 



In general, the swimbladder is found in all teleost fishes, 

 though it is without trace in the cartilaginous fishes. There are 

 some notable exceptions among bony fishes which are not un- 

 expected in the light of its normal function. It is not found, for 

 instance, in fish which spend most of their time on the sea bottom 

 and rarely rise above its surface. These include many inshore 

 fishes such as gobies, bullheads and the dragonet. Plaice, sole, 

 flounders and other flatfishes also have no swimbladder. 

 Another group in which it is absent are fishes living in turbulent 

 rivers and mountain streams, but perhaps the most surprising 

 group are fast ocean-swimming fish such as the mackerel and 

 some tunny. In these forms, which keep swimming to maintain 

 their respiratory current, it has been calculated that the absence 

 of a swimbladder decreases the muscular effort required during 

 swimming as well as removing any limitation on their vertical 

 migration. 



(i) Structure of the Swimbladder 



The structure of the swimbladder is fairly uniform and the 

 basic layers of the alimentary canal can be recognised. Two 

 layers, tunica externa and tunica interna, are usually distin- 

 guished. The outer layer is often highly extensible and consists 

 of a trellis of elastic fibres. The tunica interna has no elastic 

 fibres but contains collagen and smooth muscle. Other features 

 of the blood supply and structure of the swimbladder are shown 

 in fig. 10. In most swimbladders, two distinct vascularized 

 regions are recognised, one of which is ventral and called the gas 

 gland. Dorsally and posteriorly is found the oval, so called 

 because of its shape when viewed from inside the bladder. It is 

 now established that the gas gland is concerned with secretion 

 of the swimbladder gas and the oval with its re-absorption into 

 the blood stream. During secretion the absorptive epithelium 

 of the oval is removed from contact with the contained gas by 

 the contraction of a sphincter muscle (fig. 10a). 



Secretion of gas into the bladder is a complex and incom- 



