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 vertebræ 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.
All turtles lay eggs, the shells of which are white and generally of a parchment-like character. They are deposited in the ground or in the sand, and hatch either by the warmth of the decaying vegetation or by the heat of the sun. In temperate countries the eggs remain through the winter, and the little turtles do not emerge until the spring. The eggs of turtles are very good to eat, and the oil contained in them is put to many uses. In all the countries which they inhabit, young turtles have a hard time of it; for thousands of them are devoured by storks, alligators, and fishes. Even old turtles have many enemies, not the least strange being jaguars, which watch for them, turn them on their backs with a flip of the paw, and eat them at leisure—on the half shell, as it were!
Leathery turtles—which live in the sea—have been reported weighing over a thousand pounds! This species is very rare, and a curious circumstance is that only very large adults and very small baby individuals have been seen, the turtles of all intermediate growths keeping in the deep ocean out of view.
Snapping turtles are among the fiercest creatures in the world. On leaving the egg their first instinct is to open their mouths and bite at something. They feed on almost anything, but when, in captivity they sometimes refuse to eat, and have been known to go a year without food, showing no apparent ill effects. One method which they employ in capturing their food is interesting. A snapping turtle will lie quietly at the bottom of a pond or lake, looking like an old water-soaked log with a branch—its head and neck—at one end. From the tip of the tongue the creature extrudes two small filaments of a pinkish colour which wriggle about, bearing a perfect resemblance to the small round worms of which fishes are so fond. Attracted by these, fishes swim up to grasp the squirming objects and are engulfed by the cruel mouth of the angler. Certain marine turtles have long-fringed appendages on the head and neck, which, waving about, serve a similar purpose.
The edible terrapin has, in many places, become very rare; so that thousands of them are kept and bred in enclosed areas, or “crawls,” as they are called. This species is noted for its curious disposition, and it is often captured by being attracted by some unusual sound.