92 SUPPLEMENT ON 



labyrinthiform cavities in the head, and they often have be- 

 sides peculiar swim-bladders. 



Secretions. The secretions of fish are performed through 

 the skin, or through special secretory organs. The kidneys, 

 ureters, and bladder, with its external opening behind the 

 vent, the reverse of what obtains in quadrupeds, perform the 



* 



urinary functions. Different humours elaborated in parti- 

 cular vessels, opening externally, moisten the skin in different 

 places. In general it is a mucus not easily diluted in water. 

 In rays, there is for this purpose a great vessel surrounding 

 the nose, with several branches, and a white receptacle or 

 bag at each angle of the branchioe, from whence other vessels 

 radiate to distant points of the skin. Sharks have the snout 

 filled with a kind of mucilage, and vessels leading from 

 thence along each side of the body. Gadi, eels, &c. are 

 equally provided with similar vessels. The lateral line 

 usually is possessed of a secretory structure, operating 

 through the tubes which perforate the scales of that part. 

 One of the most remarkable secretions is the air found in the 

 swim-bladder, which in numerous genera has no external 

 communication. The organ is composed of a very fine inter- 

 nal tunic, and of an external coarse and fibrous one. It is 

 inclosed within the general coating which the peritoneum 

 spreads over all the viscera. It is variously formed, simple, 

 forked, divided into two, or even three compartments ; sup- 

 plied with several appendices, and opening into the intestinal 

 canal, the oesophagus, or the stomach. It contains in ge- 

 neral azote, mixed with particles of oxygen or carbonic acid, 

 and in fishes habituated to live at great depths, the oxygen 

 has been found so high as forty, and even, in one case related 

 by M. Biot, as eighty-seven per cent. 



The most obvious use of the swim-bladder is to keep the 

 animal in equilibrium with the water, or to increase or reduce 



