72 INTRODUCTION. 



performed independently of the mouth ; and those 

 species which can exist for longer periods than 

 usual out of their native element, have the power 

 of retaining a portion of water in a membranous 

 sack or bag surrounding the gills, which keeps 

 the filamentous structure moist, and enables the 

 animal to continue the respiratory action. Such 

 is the case with a very singular fish, the Doras 

 costatus, a native of Demerara, which possesses 

 the singular property of deserting the water, and 

 travelling overland. In those terrestrial excur- 

 sions, large droves are frequently met with during 

 very dry seasons, for it is only at this season that 

 they are compelled to this dangerous march, 

 which exposes them as a prey to so many and 

 such various enemies. When the water is leaving 

 the pools in which they commonly reside, they 

 simultaneously quit the place, and march over- 

 land in search of water, travelling for a whole 

 night in search of their object. " I have observed," 

 adds Dr Hancock, " that their bodies do not get 

 dry like those of other fishes when they are out 

 of the water ; and if the moisture be absorbed, or 

 they are wiped dry with a cloth, they have such 

 a power of secretion, that they become instantly 

 moist again. Indeed, it is scarcely possible to 

 dry the surface while the fish is living." * 



* Dr Hancock, ZqoL Journ. No. XIV. p. 242 



