12P> PROCEEDINGS OF THE ACADEMY OF [1885. 



into the head. And in certain instances, the ductless bladder is 

 divided Ity constrictions into two or three compartments, in the 

 Longitudinal direction. In these cases, the fish may have the 

 power to shift the gaseous contents of the bladder forward or 

 backward at will, and thus, by a variation in the weight of the 

 different regions of the body, to change its line of motion from a 

 horizontal to a more or less inclined direction. Yet such a func- 

 tion cannot be of any absolute importance to the fish, or 

 preparation for it Avould be far more general than we find it. 



I f we consider the conditions under which the air-bladder exists 

 in fishes, it becomes exceedingly doubtful that it was originally 

 evolved as a gravity-organ. In one important order of fishes, 

 the Elasmobranchs, it does not exist. No shark or ray possesses 

 this organ, in the main body of the fish tribe, the Teleostean, 

 its occurrence and character are very irregular. In those which 

 possess it, it exhibits an extraordinary variation in shape, size 

 and relations to the body, and this sometimes between closely 

 related genera and species. With some Teleosteans the air- 

 bladder has an open pneumatic duct, connecting with the 

 oesophagus, or in a few cases with the stomach. With others 

 this duct exists, but its cavity is closed. In some cases it is 

 reduced to a fine ligament. In many others no trace of it exists. 

 The air-bladder itself is a hollow sac, composed usually of two 

 tunics, and compressible, in whole or in part, by the aid of 

 muscles on its external surface, or by other means. It is situated 

 in the abdominal cavity, above the intestinal canal, and outside 

 the peritoneal sac, its ventral surface being invested by a fold of 

 the peritoneum. In some fishes it is almost loose in the 

 abdominal cavity. In others it is intimately adherent to the 

 vertebral column and the abdominal tissues. In many cases it is 

 inclosed in osseous capsules formed lry the vertebrae, which 

 -ccin capable of exerting a pressure upon it. In addition to the 

 cases of its longitudinal division into chambers, it is sometimes 

 composed of two lateral divisions, and in some families there is an 

 extraordinary development of lateral appendages. 



Its occurrence is as irregular as it- shape and relation to the 

 body. In this respect, it varies remarkably in species of the 

 same genus. Thus the mackerel has no air-bladder; yet one 

 exists in Scomber pneumatophor us, a species which in every other 

 respect very closely resembles the mackerel. So Polynemus 



