The typical mode of digging a burrow is described and some problems discussed which are closely connected with the burrowing instinct.The choice of the spot where the digging is started is determined by very many factors. Phototaxis and thigmotaxis are not sufficient explanations.The end-chamber of the burrow functions as an air-chamber during the high tide.The fiddler crab is a true water-breathing animal, but it can live in the air for several weeks without changing the water in the gill-chambers.The length of the burrow partly depends on the degree of moisture of the ground.There is no intrinsic periodicity in the life of Uca.Several modes of closing the burrow and opening it are described.In the interpretation the plasticity of activities is strongly insisted upon.