THEIR GENERAL CHARACTER AND STRUCTURE 94 
branchia 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 
* Tt 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 whichis 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. 
