206 THE SWIM-BLADDER OF FISHES 



open duct with the gullet, in almost all embryo fishes and in so 

 many adult forms, with the frequent presence of a blood-vascular 

 network, natunxlly suggested a respiratory use. It is not 

 surprising that a respiratory function has been v^ery commonly 

 attributed to it. ]\Iany fishes are known to swallow air at 

 times. The stone-loach (Cohitls) habitually passes air through 

 its richly-vascular alimentary canal, as does the West Indian 

 Callichthys. After oxygenating the blood circulating in the 

 walls of the stomach and intestines, it escapes in bubbles 

 posteriori}^. The sea-raven (Heinifripteriis) also distends its 

 stomach with air, while carp and similar fish, in foul or muddy 

 water, swallow air in quantities. Professor Alexander Agassiz 

 pointed out that Lepidostetis when f inch long, during the 

 second or third week after hatching, rises to the surface of the 

 water to swallow air, as it continues to do in adult life. Wilder 

 observed the same habit in the bow-fin (Anila) of the Great 

 Lakes, a regular exhalation and inhalation of air, after the 

 manner of salamanders and tadpoles, which come to the surface 

 of the water for air with increasing freciuency as the larval 

 branchiae shrink and disappear. In certain Teleosteans, such 

 as the Labyrinthici, where this resort to respiration by means 

 of the walls of the alimentary tract might be readily anticipated, 

 there is ins^'-ead a special organ, which develops in an accessory 

 branchiae cavity (Plate 23, fig, 7). Strangely enough, when 

 these fish such as Ophiocephalus and Anahas are no longer in the 

 water and are compelled to breathe air, tlie (closed) swim-bladder 

 is not even then utilised ; but the vascular laminre of the supra- 

 branchial cavity are relied upon. Again, the Globe-fishes 

 (Gymnodontes), which have the habit of distending their bodies 

 by inflation so that the spines, studding their integument, 

 project on all sides as a formidable armour, do not use the 

 swim-bladder as one would expect ; but either " inflate a sub- 

 (esophageal sac (which has a muscular sphincter, and extends 

 beneath the skin of the abdomen) rendering themselves balloon- 

 like," as Professor Macalister says (No. 14, p. 85), or fill the 



