252 PISCES. 



compared to a glottis, but this analogy will not hold good as re- 

 gards the laryngeal aperture of the higher Vertebrata, for the opening 

 of the swimming-bladder is usually found in the dorsal wall of the 

 gullet, and sometimes in its side, as in the Erythrini ; in Polypterus, 

 however, the two lateral swimming-bladders open by a common slit 

 of considerable size into the ventral walls of the gullet, so that here 

 indeed their resemblance to lungs becomes more striking. In many 

 Fishes, e. g., Mursena and Gadus Callarias, the tubes are connected 

 with the oesophagus, but there terminate blindly ; a fact which is the 

 more remarkable, since the swimming-bladder is first manifested 

 during development as an eversion, like the lungs, from the oesoph- 

 agus. 



A very remarkable union of the swimming-bladder with the or- 

 gans of hearing had been long ago detected in Heterobranchus, all 

 the species of Cyprinus, Silurus glanis, Cobitis, Clupea, and others, 

 and has been recently found to exist also in the Erythrini. This 

 union is in some cases, as in Cobitis, effected by means of the audi- 

 tory ossicles ; in others, as in Clupea and allied genera, large air- 

 canals are given off from the swimming-bladder and enter the laby- 

 rinth. 



In many Fishes we find a red Vascular gland interposed between 

 the two coats of the swimming-bladder, usually in its inferior region. 

 This gland has been falsely compared to the thymus gland, and in 

 this way the supposed analogy of the swimming-bladder to the lung 

 has been further exaggerated. But the gland in question presents 

 much more the character of a rete mirabile, and agrees in this 

 respect with similar plexuses formed by the portal vein and choroid 

 gland. It consists of a double plexus of arteries and veins, and 

 these plexuses occur in many swimming-bladders, whether provided 

 or not with an air-tube. They extend over the whole swimming- 

 bladder in the Cyprini, so that in these Fishes we find no local con- 

 centration of vessels, and consequently no true vascular gland. The 

 arteries of this organ arise from the branchial veins, while its veins 

 enter those of the* body generally. 



One or several pairs of muscles, arising usually from the trans- 

 verse processes of the adjacent vertebrae, are inserted into the swim- 

 ming-bladder of many Fishes, and appear destined to compress that 

 organ, and thus condense the air contained within its cavity. In 

 several Siluroidae, and probably also in other Fishes ( e. g., Ophidi- 

 um), a remarkable apparatus has been discovered npon the swimming- 

 bladder, which probably serves to rarefy or condense this air. Thus 



