_Dwellings hollowed out in wood._--It is not only the soil which may serve for retreat; wood serves as an asylum for numerous animals, who bore it, and find in it both food and shelter. In this class must be placed a large number of Worms, Insects, and Crustaceans. One of these last, the _Chelura terebrans_, a little Amphipod, constitutes a great danger for the works of man. It attacks piles sunken to support structures, and undermines them to such a degree that they eventually fall. Wood is formed of concentric layers alternately composed of large vessels formed during the summer, and smaller vessels formed during the winter. The latter zones are more resistant, the former are softer. When one of these Crustaceans attacks a pile, it first bores a little horizontal passage, stopping at a layer of summer-growth. It there hollows a large grotto, leaving here and there pillars of support. It lays in this space. The new generation working around the parents increases the space and feeds on the wood removed. A second generation is produced, and the inhabitants become pressed for space. The new-born pierce numerous passages and penetrate towards the interior of the pile as far as the next summer layer. There they spread themselves, always boring; they construct new rooms like the first, and arrange pillars here and there. Their descendants gain the subjacent zone, and so the process goes on. During this time the early ancestors who hollowed the surface dwellings have died, and the holes which they made are no longer habitable; but they have all contributed to diminish the resistance of the wood, and this continues as long as the race which they produced makes its way towards the centre of the stake. [Illustration: FIG. 22.]