A turtle cannot expand its chest at all, it must suck in air by moving things about inside. By contracting a pair of muscles back at the flanks, the turtle can increase the volume of the space around the lungs, and the air rushes in. To expel air, it contracts another set of muscles under the viscera, pushing the internal organs forward to squeeze against the lungs. A number of other physiological adjustments makes breathing a distinctive process in turtles and, incidentally, one about which a great deal remains to be learned.
Turtles range in weight from a few ounces to well over half a ton. The biggest turtles are aquatic, but there are big ones on land too. The famous Galapagos tortoises, and others on islands in the Indian Ocean, have reached weights of over 400 pounds. During the Pleistocene, there were even bigger ones in various parts of the world.
There is a popular notion that turtles live, as it were, forever. Little is known scientifically about their maximum life span, however, and it is not possible to evaluate this notion. Careful sifting of records from zoos , and of the generally shaky evidence afforded by turtles with dates into their shells, has led some herpetologists, as students of reptiles are called, to suggest a figure of a hundred years as a probable maximum. Few turtles living near man realize this potentiality. They get run over on the road, their marshes or ponds are drained, their streams are poisoned, or they are simply caught and eaten. Fortunately for their survival, their longevity does not mean they are slow to mature. In fact, turtles reach sexual maturity in a surprisingly short time. In the several species for which data are available, including some of the big sea turtles, breeding may begin at ages of from three to eight years.
The most distinctive of living turtles is the giant leatherback sea turtle. Though it looks like a turtle, it has no proper turtle shell, but only a rubbery skin covering a mosaic of little, pebblelike bones which have nothing to do with the broad bones of the ordinary turtle carapace and are not connected to the skeleton at all.While the four other genera of sea turtles show some modification of the bony carapace, their shells are more like those of land and fresh water turtles. Another turtle group that stands apart is that of the soft-shelled, or pancake, turtles, distributed in Africa, Asia and North America. In these the horny shell is also replaced by a continuous skin, and the edge is thin, floppy cartilage with no supporting marginal bones.
The motley array of the remaining kinds of turtles falls into two main groups, according to the way they draw in their necks. Most of them, the Cryptodira (hidden necks), retire with the neck bent into a vertically folding, S-shaped curve. They are found in Europe, Asia, Africa and the Americas. The other group, the side-necked turtles (Pleurodira), brings the neck in sideways and lays it along the body under the fore eaves of the shell. Side-necks are confined to tropics of the southern continents, Africa, South America and Australia. One of them is the famous matamata (Chelys fimbriata), the most grotesque-looking of all the turtles. The matamata, in fact, does not look like a turtle at all; it looks like a pile of leaves. In Columbia, where it is fairly common in swampy streams, and where a particular region contains a strain of extremely homely Indians, the women of this group are sometimes referred to as having cara de matamata, the face of a matamata. I judge that this is not said to their faces.
From the standpoint of abundance and diversity, the lizards and snakes are by far the most flourishing reptiles of today. Between them the two groups include about 600 genera and at least 5,700 species.
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