104 



llie other gets filled as well, so that the course and communication of 

 the arterial and venous vessels can readily be made out. 



In the gurnard the air-bladder is short and very muscular ; it has 

 no ductus pneumaticus, but, like that of the cod, is provided with a 

 gland, and with the same parallel distribution of vessels : the same 

 structure also is found in all the fish which I have had an opportunity 

 of examining, these now amount to upwards of thirty species. 



Having said thus much on the peculiar arrangement of the capilla- 

 ry vessels in the organ in question, it now remains for us to consider 

 whether any light can be thrown upon the function which the air- 

 bladder performs, from an inspection of its blood-vessels. To enu- 

 merate all the authors who have written on this subject, would be a 

 task of no small moment, and for our present purpose such a recital 

 would be useless. Suffice it to say that the talents of a Cuvier and a 

 Hunter have been employed in the investigation, and the true use of the 

 air-bladder, to the present day, remains as an unsolved problem, and 

 the points at issue are these, whether it acts only the part of a float, or 

 whether it is at all concerned in the function of respiration. If all 

 fishes were supplied with an air-bladder, the question could be settled 

 without much difficulty, but such is not the case. Mr. Yarrell, in his 

 * History of British Fishes,' informs us, that " one fourth of the fishes 

 known have no air-bladder at all, and that two thirds of the other three 

 fourths have neither canal nor aperture for external communication." 

 Besides this, the air-bladder is said to be present in one species of 

 a genus and absent in another. Our common mackerel {Scomber 

 Scomber), is said to have no air-bladder, whilst of the same genus, 

 Scomber pneumatophorus has one. Many other instances of this kind 

 could be mentioned. The flat fish, and most of the cartilaginous fish- 

 es, are without air-bladders, while most of those which live near the 

 surface of the water are supplied with them. Others on the contrary 

 are supplied with air-bladders which are sacculated, and extend near- 

 ly the whole length of the body, precisely like the lungs of the ser- 

 pent tribe, and the ductus pneumaticus which opens into the pharynx, 

 is even surmounted by a perfect glottis, and provided also with mus- 

 cles to close it. The best example of this kind of air-bladder is found 

 in a fish of the pike kind, named the Lepidosteus osseus. It inhabits 

 some of the rivers in the interior of North America, and is the only 

 living representative of a long-lost family of bony-scaled fishes. Pro- 

 fessor Agassiz, in dissecting one of these fish, was immediately struck 

 with the great analogy between its air-bladder and the lung of a ser- 

 pent ; and this organ has also been made the subject of a paper in 



