322 PHILOSOPHY OF ZOOLOGY. 



at opposite sides of the hole. These are considered by 

 Monro, as having the effect of a sphincter muscle. In the 

 salmon, the last quoted author found a hole so large as to 

 admit readily the largest sized goose-quill, leading directly 

 through the coats of the oesophagus into the air-bag. The 

 oesophagus in this fish has a thick muscular coat, but the 

 fibres of that coat do not seem to form a distinct sphincter 

 around the hole. In other fishes, the duct of communica- 

 tion is of considerable length. In the common herring, 

 the under part of the stomach has the shape of a funnel ; 

 and, from the bottom of the funnel, a small duct is pro- 

 duced, which runs between the two milts, or the two roes, 

 to its termination in the middle of the air-bag. In some 

 fishes, as the cod and haddock, Moniio could not perceive 

 any ductus pneumaticus, or opening into any of the abdo- 

 minal viscera. The air-bag was not enlarged by blowing 

 into the alimentary canal, nor could he empty the air-bag 

 without bursting it. 



In the air-bag of the cod and haddock, the same acute 

 observer examined the red-coloured organ noticed by Wil- 

 LOUGHBY, and considered by him as a muscle, the surface 

 of which is very extensive, as it is composed of a vast num- 

 ber of leaves or membranes doubled. In those fishes, how- 

 ever, in which the air-bag communicates with the alimen- 

 tary canal, this red body is either very small and simple in 

 its structure, as in the conger eel, or entirely wanting, as 

 in the sturgeon, salmon, herring, and carp. 



Naturalists, in general, are disposed to regard the air- 

 bag as accessory to the organs of motion. Having observed 

 that flat fish, which reside always at the bottom, are in ge- 

 neral destitute of this organ, they have assigned to it the 

 office of accommodating the specific gravity of fishes to the 

 density of the surrounding element, and thus enabling 

 them to suspend themselves at any depth. A very simple 



