Chelonia

The Sources of Tortoise Shell and Terrapin Stew

The turtles and tortoises are of a very ancient group (Chelonia) and one very distinct among reptiles, by reason of their armor. What is known as tortoise shell is the series of horny plates, in some species of beautiful texture, in others thin and dull, or even leathery in character, that covers the underlying bones that form the real protection to the animal's body. In embryo (unhatched) turtles the skeleton is much like the ordinary four-footed type, with the vertebræ separate, a full series of ribs, and the limb bones in their proper places. As growth proceeds, however, changes occur rapidly, but least in the oceanic "leathery" turtle, in whose skin nodules of bone expand and join into a mosaic of plates covered with a thick, coriaceous hide. But this skin remains quite separate from the skeleton beneath, which fact places this animal in an order Athecæ ("lacking a case"), quite by itself. All other chelonians are classified in a second order Thecophora ("case-bearing"), and in them the changes that go on in the skin to produce the turtle's shell are far more complete.

If you peel off the horny shields on the upper shell, or "carapace," you will find beneath them a central, lengthwise row of squarish plates of bone, on each side of these a row of similar plates, and outside of these a marginal row of small plates—all knit together at the edges, the zigzag lines of juncture, or "sutures," being plainly visible.

When we dissect a turtle we find no layer of skin or flesh beneath these plates, but discover that they lie directly on the bones of the skeleton and are a part of it. This is what has happened: The vertebræ have grown together, and the backbone is a tube upon which the original nodules in the skin have become fixed, and have broadened into the central line of plates. Those nodules that lay above the ribs have become fused with them so that no trace of ribs is left, except where their heads have become fused with the backbone, and they have broadened into the side rows of plates; and the marginal skin has become transformed into the marginal plates. Similar alterations have produced the under shell, or "plastron," replacing the skin; and adaptive changes have altered the usual relations of the limb bones to the rest of the external skeleton. The carapace and plastron are usually connected by a "bridge" of bone.

Into the space within this shell most tortoises may withdraw the head and tail which, like the feet, are covered with horny scales. The head has good eyes, and a nose with a lively sense of smell, which the creature utilizes in selecting its food; but its hearing appears to be dull. The mouth has no teeth, but the lips are coated with horn, making a parrotlike beak that can inflict a severe bite. Horny spines often grow on the legs, or tail, or both, assisting in both defense and offense.

The chelonians are a very ancient race, and one that has changed remarkably little since its beginning. The great age accounts for the very wide distribution of turtles closely related, and also for the fact that they inhabit land, fresh and salt water; those of the land being, no doubt, the oldest. All turtles lay eggs, the shell of which varies, according to kind, from a parchmentlike envelope to a hard, shining shell; but the process of generation is slow and curiously complicated.

The respiration of the Chelonia is interesting. The lungs are spongy masses, attached to the upper shell. As the rigid case does not permit of their expansion by breathing, the necessary vacuum is made partly by the neck and limbs, which act like pistons as they are drawn in and out, the air being swallowed or pumped into the lungs. Most chelonians may exist for a very long time without breathing, and can stay for hours or even days under water. No animal, perhaps, is harder to kill; and all turtles have long lives, the giant turtles of the Galapagos and their kin living more than 100 years.

The list of Thecophora begins with the suborder Cryptodira, whose members have the carapace covered with horny shields, and consists of the family Chelydridæ, composed of our two snapping turtles, the familiar northern one, and the southern alligator snapper. They inhabit stagnant pools, especially deep channels in swamps and slow rivers such as the bayous of the lower Mississippi Valley, and often show only the tip of the nose as they prowl about close beneath the surface in search of prey—anything they can seize. They take the hook readily if baited with fish or flesh, but stout tackle and a strong arm are needed to land one when full grown; and the act is dangerous to the catcher, for they are the ugliest brutes in the country, and to be bitten by one is a very serious experience. Nevertheless, the young are caught for market in large numbers, for they are excellent food. One curious fact about them is not generally known, namely, that when lying still, like a piece of old log covered with mud and moss, they protrude a pair of wormlike filaments from the tip of the tongue, whose wavering attracts fishes to their doom. Louis Agassiz says of the great alligator snapper of the Southern States, which when walking on land carries its body high on the long legs, much like an alligator:

"They are as ferocious as the wildest beast of prey, but the slowness of their motions, their inability to repeat the attack immediately, their awkwardness in attempting to recover their balance when they have missed their object, their haggard look, and the hideous appearance of their gaping mouth, constitute at such times a picture as ludicrous as it is fearful and revolting. Their strength is truly wonderful. I have seen a large specimen bite off a piece of plank more than an inch thick.... Fishes, salamanders and young ducks are their ordinary prey. They lay from twenty to forty or more round eggs, only about the size of a small walnut, in holes which they dig in sloping banks not far from the water."

These snapping turtles probably represent well the disposition and habits of the extinct predatory reptiles; and give us a hint of why the race succumbed to the more active and intelligent mammals that were growing up around them toward the close of the Mesozoic.

This brings us to the great family Testudinidæ, which is scattered over the whole world except Australia, and contains almost all the ordinary tortoises, mud turtles and terrapins, some of which are entirely aquatic, others amphibious, others wholly terrestrial. Among the most typical and widely distributed are those of the genus Chrysemys, to which "mud turtles" belong. The commonest species in the Eastern States is the painted turtle Chrysemys picta ; in the West, C. marginata. These and other species of North and South America are very pretty when young, the ground color of the upper shields being green, variegated with yellowish or blackish markings, often in delicate patterns. They are carnivorous, depend mainly on fish, but eat many insects and their larvæ. In winter they hibernate in holes in a bank of their pond.

TERRAPIN
(Malaclemys palustris )
(After Babcock. Boston Society of Natural History)

To the genus Clemmys is credited the "sculptured" wood tortoise, the keeled plates of whose back are marked with fine concentric grooves and radiating black lines; and the equally common speckled one, black with round, orange spots. Both spend long periods wandering in woods and fields in search of worms and insects. Closely related to them are the salt-marsh tortoises known as terrapins, which are so much of a luxury in the eyes of those fond of good dinners that probably the favorite one, the"diamond back," would be extinct had not protective measures, and cultivation in captivity, saved the life of the species. Several other terrapins are to be found in the marshes of the Southern States, but not in other countries. Another relative is the interesting little "box tortoise," which is often kept as a pet, and will become very tame; its highly convex shell is colored black and yellow, or orange-brown, but no two are alike. The eyes of the male are red, those of the female yellow. It is naturally enough a "box" tortoise, for, by means of a flexible joint line across the plastron, the fore and hinder halves can be brought up to the ends of the carapace, shutting the whole body inside a tight box that will defy all enemies not strong enough—as are wolves, bears, and big cats—to tear it to pieces. This tortoise is exclusively American. It has become, as a species, wholly terrestrial, so much so that, although it is fond of drinking often, if it falls into the water it will drown. It thus leads us to the true land tortoises that fill the remaining genera of this family.

BOX TORTOISE
(Cistuda carolina )
(After Babcock. Boston Society of Natural History)

The typical and most numerous of these belong to the genus Testudo, with about forty species scattered over all warm or temperate parts of the world except Australasia. Typically grazers and fruit eaters, they occasionally vary their diet with worms, snails, and insects. The eggs are hard-shelled, and the males are usually smaller than their mates. Most land tortoises hibernate in the ground during the cold half of the year, or they æstivate during the hot and dry seasons when in the tropics, but this is not an invariable rule. Several species of these land tortoises are common and well known in Europe and also in India, and are often kept as pets. They show considerable intelligence, and are decidedly fond of listening to music. Our best known American representative is the "gopher" (Xerobates polyphemus ) of Florida, Georgia, and Texas. This turtle is nearly a foot long, with a high, rounded shell, dull brown in color, and the forefeet covered with hornlike scales and some spines—an armature for digging. The deserts along the Mexican border have several local species.

In this family belong the "gigantic" tortoises of the islands east of Africa and west of South America, now all but extinct, save a few in captivity in zoölogical gardens. In fact they differ from ordinary land tortoises mainly in size and in such minor points as distinguish the various species; some of them, indeed, are not excessive in bulk. The largest on record is a male of T. daudini, of South Aldabra, whose shell was sixty-seven inches long, and whose living weight was 500 pounds. A fossil species of the late Miocene in India had a shell six feet long, and then and later tortoises almost as big inhabited both Europe and North America, and more recently Madagascar. Their survivors are now restricted to two widely separated regions—the Galapagos Islands, 600 miles off the coast of Ecuador, and the Mascarenes and other western islands in the Indian Ocean. The most interesting thing about this matter is the presence of these tortoises on these widely scattered islands, and the effects of their isolation. It must be noted that when discovered by European voyagers no one of these islands, except the Comoros, was inhabited by men, and none had any large or harmful beasts of prey.

On these peaceful islands plenty of food, an equable climate, and absence of enemies, enabled the tortoises in vast numbers to grow to a size impossible to their relatives on the mainlands. "Scattered over the many islands they were prevented from interbreeding, and thus it has come to pass that not only every group of islands, but, in the case of the Galapagos, almost every island has, or had, its own particular kind." How did these huge chelonians get to these islands? None like them is found on any continent at present, although they had a wide distribution in geological ages. We must conclude that those of the Madagascar region, at least, are the descendants of tortoises once populating "Lemuria," that land area which until mid-Tertiary time occupied the region of the western Indian Ocean, and of which the existing islands are the remains. A similar theory, for which there is geological evidence, may account for the survival of the Galapagos giant tortoises after those of the mainland had died off.

The next family is that of the big sea turtles (Chelonidæ), such as the green turtle, whose flesh is so highly prized a substance for delicate soups (but almost all turtle flesh is good eating), the hawksbill and the loggerhead. They abound in all warm seas, and reach a large size, the green turtle often having a shell three to four feet long, but smooth, while that of the hawksbill is covered with horny plates with high keels and an overlapping arrangement, which are the tortoise shell of commerce. The green turtle is wholly vegetarian in diet, feeding on the large seaweeds, while the others are carnivorous, devouring fishes, mollusks, etc. All three resort in summer to sandy beaches, dig holes, and bury a great quantity of eggs.

There remains a large group of fresh-water turtles, distinguished, in addition to other important structural peculiarities, by the fact that they withdraw the head under the shell by a sidewise bending of the long neck. They are entirely carnivorous, and occur in all tropical and some temperate countries, the ferocious "soft shells" of the Mississippi Valley and northeastward belonging here. A very curious one is the matamata of Guiana and northern Brazil, the biggest of its tribe, becoming more than three feet long. It gets its living by stratagem rather than by activity. The back of its shell is so roughened by coarse bosses that it looks like the bark on an old log, and ragged flaps of skin project from head and neck. These are kept in constant motion, and attract the attention of passing fishes and other creatures, whose curiosity often takes them too near the treacherous jaws of the concealed monster.

Our list of turtles ends with the one probably of most importance to mankind of all the kinds in the world. This is the "arrau" of the Amazon and Orinoco basins, where it is very abundant, and not only an essential element in the subsistence of the native Indians, but of great commercial importance on account of the eggs, which are periodically collected in enormous quantities, chiefly for their oil. This oil is eaten, like the eggs themselves, or is used for burning in lamps, or as an addition to tar. The turtles are likewise eaten by man and beast. This turtle is large, sometimes three feet long; and it deposits a great number of soft-shelled eggs in the sand.