For many centuries the lagoons of Venice have been divided into districts for the purposes of fishing. These tracts of water are not distinguished by any boundaries visible to the eye; but their limits are well known to the fishermen who make their living upon them. In the shallower parts, where the oozy bed of the lagoon is left bare by each receding tide, the fishermen mark off a certain portion, and surround it by a palisade of wattled cane called a grisiola. Inside this palisade the mud is dug into deep ditches, so that there shall always be water in them, even when the tide is low.    These enclosures are called valli and here the fish are driven in spring to spawn. Each valle has a little hut belonging to it, built either on piles or on forced soil, and made of bricks or of wattled cane, plastered with mud. The hut usually contains one square room, a door, and two windows. The fishermen require these cabins, for they sometimes spend three or four days together in the rem...