The toes are usually provided with membranes spread between them, to assist in swimming. The form of the tail, which is generally compressed vertically, like that of fishes, though perhaps not to an equal degree, is another indication of their being formed for an aquatic life: for where the tail has this shape, we always find that the chief muscular power is bestowed upon it as an instrument of aquatic progression, producing, by its lateral flexions, a horizontal movement of the body. Crocodiles and alligators, for instance, which have this conformation, are comparatively weak when on land, and as soon as they have seized their prey their efforts are always directed to drag it with them into the water; knowing that when in their own element they can readily master its struggles, and dispose of it as they please. In the Gecko tribe we find a particular mechanism provided for effecting the adhesion of the feet to the objects to which they are applied. It is somewhat analogous to that employed in the case of the house-fly, already mentioned. Each foot has five toes; all, except the thumb, terminated by a sharp curved claw. On the under surface of each toe there are as many as sixteen transverse slits, leading to the same number of cavities, or sacs; these open forward, and their external edge is serrated, appearing like the teeth of a small-toothed comb. All these parts, together with the cavities, are covered or lined with cuticle. Below them are large muscles which draw down the claw; and from the tendons of these muscles arise two sets of smaller muscles, situated so as to be put upon the stretch, when the former are in action. By the contractions of these muscles the orifices of the cavities, or sacs to which they belong, are opened, and the serrated edges applied accurately to the surfaces with which the feet are in contact. Sir Everard Home, in his account of this structure, compares it to the sucking disk of the Remora. By its means the animal is enabled to walk securely upon the smoothest surfaces, even in opposition to the tendency of gravity. It can run very quickly along the walls or ceiling of a building, in situations where it can not be supported by the feet, but must depend altogether upon the suspension derived from a succession of rapid and momentary adhesions.