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The Air vessel consists of three coats or coverings lying closely one over 

 the other. The interior one is a thin serous lining membrane, by which the 

 air is secreted. Next to this is a vascular membrane full of conspicuous blood- 

 vessels, which supplies the blood from which the air is secreted by the inner 

 membrane. The thh-d and outer covering is a strong fibrous coat, which serves 

 to protect the apparatus from injury by accident or sudden movements : it ia 

 also often muscuLir, giving the fish power to contract the vessel at will. 



It is the opinion of some jihysiolegists that the Air bladder in its most 

 simple form is developed as a process or diverticulum from the upper part of the 

 alimentary canal, so that when it forms a closed sac the original communication 

 must have been obliterated. However this may have been, we find in the large 

 majority of fish possessing air bladders that the sacs are closed on everyside, and 

 have no commiinication with any other organ. This is the case ia the order of 

 Acanthopteri (Midler), which includes among other kinds the Perch, and one 

 kind of Mackerel, and also in the Gadidse branch of the Anacanthini, consisting 

 of the Cod, Haddock, Turbot, Ling, &c. The air bladder of the Cod is of large 

 size, commonly called the sound, and is covered with thick coats. It is entirely 

 destitute of any communication with the stomach or gullet, as we shall find to 

 exist in those of nearly all fresh water fishes. This is also the peculiarity of 

 the orders of Pharjiigognathi and Plectognathi, consisting almost entirely of 

 sea fish. On the other hand, we find that all the fish of the order of Malaoopteri, 

 Buch as the Herring, Salmon, Trout, and the Salmonid;e generally. Pike, and the 

 Eel, possess a communication between the air vessel and some part of the 

 alimentary canal near to the stomach by a short duct called ductus pneumaticus ; 

 and in the Cyprinidje or Carp tribe this duct leads to the Oesophagus or 

 gullet. A similar duct is found in the Ganoidei order, and in the British family 

 of the Acipenserid<e or Stui-geon tribe, which have very large air bladders, and 

 communicating with the gullet by a short and wide duct. 



These air vessels are, in some instance, as in the Salmon, Trout, Pike, 

 Perch and Eel, closely attached to the side and the spine of the fish, while in 

 others, as in the Carp tribe, they float loosely in the stomach. There are also 

 intermediate varieties. Their shape varies much. The ordinary simple form is 

 that of a single lobe of an elongated-oval shape, or two arcs of a circle joined 

 together. In all the Salmon, Herring, Pike, and Eel famUies, it is one elongated 

 cylintlrical tube lying close to the under surface of the backbone and adhering 

 to it, and in some cases, as in the Pike, connected with the ribs by strong 

 ligaments. Another and quite distinct form is that where there are two 

 chambers or lobes with oblong cavities, the anterior one generally sUghtly 

 truncated at the end placed one behind the other, and connected by a short 

 tubular neck. In all classes of fish where the air vessels are of this form, and 

 not being closed sacs, there is a duct leading from the anterior part of the 

 hinder lobe to the oesophagus, and in this duct there is a valve closing outwards, 

 which while it allows the aii- to be expelled prevents its admission. This is the 



