The "house" or shell of a turtle is made up of separate pieces of bone, a central row along the back and others arranged around on both sides. These are really pieces of the skin of the back changed to bone. Our ribs are directly under the skin of the back, and if this skin should harden into a bone-like substance, the ribs would lie flat against it, and this is the case with the ribs of turtles. So when we marvel that the ribs of a turtle are on the outside of its body, a second thought will show us that this is just as true of us as it is of these reptiles. This hardening of the skin has brought about some interesting changes in the body of the turtle. In all the higher animals, from fishes up to man, a backbone is of the greatest importance not only in carrying the nerves and blood-vessels, but in supporting the entire body. In turtles alone, the string of vertebrae is unnecessary, the shell giving all the support needed. So, as Nature seldom allows unused tissues or organs to remain, these bones along the back become, in many species, reduced to a mere thread. The pieces of bone or horn which go to make up the shell, although so different in appearance from the skin, yet have the same life-processes. Occasionally the shell moults or peels, the outer part coming off in great flakes. Each piece grows by the addition of rings of horn at the joints, and (like the rings of a tree) the age of turtles, except of very old ones, can be estimated by the number of circles of horn on each piece. The rings are very distinct in species which live in temperate climates. Here they are compelled to hibernate during the winter, and this cessation of growth marks the intervals between each ring. In tropical turtles the rings are either absent or indistinct. It is to this mode of growth that the spreading of the initials which are cut into the shell is due, just as letters carved on the trunks of trees in time broaden and bulge outward. The shell has the power of regeneration, and when a portion is crushed or torn away the injured parts are gradually cast off, and from the surrounding edges a new covering of horn grows out. One third of the entire shell has been known to be thus replaced. Although so slow in their locomotion and actions, turtles have well-developed senses. They can see very distinctly, and the power of smell is especially acute, certain turtles being very discriminating in the matter of food. They are also very sensitive to touch, and will react to the least tap on their shells. Their hearing, however, is more imperfect, but as during the mating season they have tiny, piping voices, this sense must be of some use. Water tortoises can remain beneath the surface for hours and even days at a time. In addition to the lungs there are two small sacs near the tail which allow the animal to use the oxygen in the water as an aid to breathing.