222 THE SWIM-BLADDER OF FISHES 



other various interpretations which have been put forth, serious, 

 and to my mind, fatal objections exist. 



In conclusion I may sum up the points on which I would 

 lay emphasis. The swim-bladder, in its earliest condition in 

 the embryo and the post-larval stages, exhibits the characteristic 

 features of a secreting gland, opening by a duct into the anterior 

 portion of the alimentary canal. It arises as a proti-usion from 

 tlie digestive tract like the hepatic evagination which becomes 

 the liver, but this huge gland (the liver) with its capacious 

 bile-duct is on the ventral side whereas the swim-bladder is on 

 the dorsal side of the tract. The degeneration and disappearance 

 of the swim-bladder in so many fishes proves that it is clearly 

 not essential for flotation. Some of the most active pelagic 

 species like the Sharks, Sun-fishes, Mackerels and others are 

 without it, while fishes like the Pleuronectidfe and other bottom- 

 frequenting species likewise lack that organ, though apparently 

 so much in need of a hydrostatic device to facilitate their ascent 

 or descent in the water. Inshore littoral fishes and shallow- 

 water species (freshwater and marine) are almost without 

 exception piovided with it, though so little needed, in their case, 

 for floating purposes. If its hydrostatic utility be questionable, 

 it is clearly not for respiratory purposes, as in most fishes venous 

 blood is not convoyed to it ; but blood which has immediately 

 before been arterialised and fully purified in the gills. Its 

 dorsal connection and position superior to the alimentary canal 

 indicates that it is not the homologue of the pulmonary organs 

 with their auct opening on the lower side of the oesophagus. 

 Professor Arthur Thomson pertinently observes, (No. 23, p. 391), 

 " The air-bladder lies dorsally and is almost always single : the 

 lungs lie ventrally, and are double, though connected by the 

 gullet by a single tube. It is not certain that these outgrowths 

 are homologous, though the air-bladder of Dipnoi acquires the 

 functions of a lung." That the swim-bladder aids in audition, 

 and in sound-production or voice, is suggested by the chain of 

 ossicles (the complex Weberian apparatus) in such fishes as the 



