Security appears still to be the object aimed at in the mechanism of all other parts of the skeleton. After the head has been drawn in by the double or serpentine flexion of the neck, the knees are brought together and the whole limb withdrawn within the shell, the forelegs folding completely over the head, so as to cover and protect it most effectually. Considerable differences may be noticed in the structure of the several species of Chelonia, according to the diversity of their habits. Tortoises which live on land require more complete protection by means of their shell than turtles, or Emydes, which dwell only in the water: hence the convexity of their carapace, the solidity of its ossification, its immovable connection with the plastron, and the complete shelter it affords to the head and limbs. Turtles, on the other hand, receiving support from the element in which they reside, require less provision to be made for these objects. Previously to the retraction of the head and limbs within the shell, the air is expelled from the large cavities of the lungs by the vigorous actions of the abdominal muscles, which exist in these animals as well as in all the vertebrata, although here they are covered by the bones, and compress the lungs by pushing the abdominal viscera against them. This sudden expulsion of air is the cause of the long-continued hissing sound which the tortoise emits while preparing to retreat into its stronghold. The ribs, though they first assume the form of broad plates immovably united to the spine, when they have proceeded a certain distance separate from each other and resume their usual form; the intervening spaces between two adjacent ribs being here filled up by membrane. The plastron is united with the carapace by membrane likewise; and the sternum, instead of forming one broad plate of bone, has the intervals between its imperfectly developed elements also membraneous. All this renders the whole shell less compact, more flexible, and more feeble: but the movements of the animal are quicker and more energetic. These characteristic differences between the aquatic Chelonia and those that live on land are still more strongly marked in the genus Trionyx, or soft tortoise, which is destitute of scales, and in which many of the pieces that are bony in the tortoise are replaced by simple cartilage or membrane.