MIGRATIONS. 127 



filled with that fluid. This trough exists in the 

 horsemen land-crabs, 1 but it is smaller, and a 

 spongy mass furnishes the requisite moisture. 

 The gills of the land-crabs, in other respects, do 

 not differ from those of the tribe in general. 

 God, when he formed these animals, would not 

 separate them from their kind by a different 

 mode of respiration, but by this compensating 

 contrivance he fitted them for the circumstances 

 in which he decreed to place them, and for a 

 long sojourn out of the water. 



What is the great object of this law of the 

 Creator, that impels them to seek, in many cases, 

 a mountain retreat, at a distance from the ocean, 

 which forms the liquid atmosphere fitted to the 

 great body of the Crustaceans, has not hitherto, 

 for want of sufficient and accurate details of their 

 history, been made fully obvious. When insects 

 leave the waters to become denizens of the earth 

 and air, the object appears evidently an increase 

 of food, not only for terrestrial animals, whether 

 moving on the one or in the other, but to mul- 

 tiply even that of the inhabitants of the waters. 

 When the day-flies 2 burst in such myriads from 

 the banks of rivers which they inhabited in their 

 first state, the fishes are all in motion, and often 

 jump from the water to catch the living flakes 

 that are every moment descending. When in the 



1 Ocypode. 2 Ephemera. 



