THEIR GENERAL CHARACTER AND STRUCTURE 9 



branchiae or gills.* Let these organs once get dry, and the 

 animal perishes of suffocation just as certainly as a lung- 

 breathing creature will do if kept under water. Some fish, 

 like the stone-loach, die almost immediately upon exposure to 

 the air ; but so careful is the directing power of Nature that no 

 part of her dominion shall be void of life, that other fish are 

 furnished with special mechanism for retaining moisture round 

 the gills. Thus that strange creature, the African Lepidosiren 

 (Protopterus\ inhabits rivers which run dry every year, a 

 catastrophe which would exterminate in a single season the 

 whole population of an English stream. But Protopterus 

 thrives under these trying conditions, attaining a length of six 

 feet, and feeding ravenously upon other animals so long as the 

 water lasts. When it fails, the fish sinks into the mud and 

 forms around itself a kind of cocoon or capsule of its own 

 slime, which hardens as the mud dries and forms a secure 

 retreat, within which the animal lies torpid until the water 

 returns with the ensuing rainy season. In this state the fish 

 are often dug out and exported in the form of clay balls to 

 zoological societies in Europe and America. Provided that 

 evaporation is not caused by cracking or breaking the capsule, 

 Protopterus may be released on arrival by soaking the covering, 

 when the inmate is found quite lively and possessed of a pro- 

 digious appetite. 



The water supply in temperate regions is not subject to such 

 complete intermission as in the tropics ; nevertheless, natural 

 gradients and artificial obstructions suffice to prevent the 



* It is to be noted that the oxygen extracted from the water by fishes is 

 not that which forms a chemical constituent thereof, but the free oxygen 

 contained in the air which is dissolved in: the water. Dr. Giinther, in noting 

 that very little oxygen suffices to purify the meagre volume of blood 

 in a fish, cites the calculation that a man consumes fifty thousand times 

 more oxygen than a tench. Now, as a man of ten stone weighs no more 

 than seventy tench of two pounds each, if these two vertebrates were 

 constituted alike, the man's requirement would be just seventy times that 

 of a single tench. 



