141 



inore closely even than in the Sabnonidce. It is also of more delicate construo; 

 tion, consisting of very fine transparent membrane, extending between the 

 vertebral column and the abdomen. This membrane forms also the coveiing or 

 lining of the ribs and spine, and so closely adheres to them that it is impossible- 

 to take out the air-vessel entire. The delicate inner Uning is plainly visible. 

 The air-vessel forms a closed sac, and there is no communication from it either 

 ^o the gullet or oesophagus. 



From the preceeding remarks the following general conclusions may be 

 ^rawn: — 



1. All fresh-water fish, with a single exception, and the generality of sea 

 fish, have air-vessels. 



2. In the large majority of fresh-water fish there is some communication 

 between the air-vessel and the intestinal canaL 



3. In the large majority of sea fish the air-vessels are closed sacs. 



4 The position of the air-vessel is always the same, near the centre of 

 the body. 



5. The air-vessel varies considerably in shape in different fish. 



6. The gas contained in the air-vessel is secreted by the lining membrane 

 pf the sac. 



7. Its constituents are the same as atmospheric air, but vaiy in their 

 proportions. In the air-vessels of fresh-water fish nitrogen is in excess ; and 

 in those of sea fish oxygen is most abundant. 



8. The chief use of the air-vessel is t« lessen the specific gravity of the 

 fish, and thus to enable it to poise or balance itself with the smallest amount 

 of muscular exertion. 



'9. Fish possess the means — more or less evidently shown — of regulating 

 the amount of air in the vessel, so as to adaiit their specific gravity to the 

 position they occupy in the water, to the temperature of the water itself, or 

 to the purposes they wish to effect. 



10. "WTiere the sac is closed, the change is probably produced by the 

 slow process of reabsorption of the air. 



11. Where the duct of communication — the ductus pneumaticus — is small 

 and long, as is most frequently the case, the change is more readily affected. 



12. When the ductus pneumaticus is short and large, a great change may 

 be instantaneously affected. 



13. The air-vessel is a rudimentary lung, adapted to fulfil the secondary 

 pbject of lessening the specific gravity of the fish. 



14. In some rare cases it may stUl form a portion of the respiratory 

 apparatus, and possibly does so in many more when an emergency calls for it. 



