Before leaving the description of the shell, it is worth while to draw attention to the enormous correlative changes in other organs produced by this case. Nearly the whole organism has been altered. The hard, firm carapace has partly rendered the supporting functions of the vertebral column unnecessary or impossible. In many tortoises, especially in the large land-tortoises, the vertebrae and the capitular portions of the ribs are reduced to mere bony outlines; the reduction to thin paper-like bony lamellae proceeds with age. The iliac bones find a better support in the costal plates; the contact with the sacral ribs is given up, and these ribs fuse partly with the costal plates, or they are absorbed. The whole mass of muscles of the trunk is completely lost in the region of the shell, but traces of them exist in young specimens. Neck, limbs, and tail can in most cases be withdrawn and hidden in the shell. When this is not possible it is due to secondary changes. The neck is withdrawn either by being tucked away sideways (Pleurodira[[129]]), or by being bent in an S-shaped curve in a vertical plane. In a left-sided profile-view of the animal, the head represents the tail of the S. The neck is withdrawn by long muscles, which are inserted into the ventral side of the middle of the neck, and extend in the shape of vertical ribbons far back into the shell, arising from the centra of some of the middle or even more posterior thoracic vertebrae.
Lastly, a few remarks on the partial regeneration, or the mending of injuries to the shell. If part of the horny covering is badly bruised, torn off, or rubbed through, or if part of the shell is crushed, the underlying portion of the bony plate becomes necrotic, and the horny covering also dies so far as its Malpighian layer is destroyed. Soon, however, the uninjured Malpighian cells, around the margin of the wound, multiply, grow into and beneath the injured portion of the bone, and form a new horny layer, casting off the necrotic portion. After several months the deficiency is patched up; new bone has grown in the deeper remaining strata of the cutis, and the outside is covered by a continuous horny layer, without, however, reproducing the original concentric moulding of the shields. In badly crushed shells sometimes almost one-third of the whole shell is thus cast off and mended within one or two years. The regeneration of the forcibly stripped-off shields of Chelone imbricata is described on p. [386]. Bitten-off tails and limbs, rather frequent occurrences in water-tortoises, are of course not reproduced, but the wounds are healed and covered again with scaly skin.
Sense-organs.–The EYE is by far the best developed sense-organ. It is comparatively small. The pupil is round. The iris is mostly dark in terrestrial forms, while in water-tortoises it is often brightly coloured, for instance pale yellow in Chelodina, greenish and mottled with black, pale grey, brown, etc., in various species of Chrysemys. Cistudo presents a curious sexual dimorphism; the males have red, the females brown, eyes. The sclerotic wall contains a ring of numerous small ossified plates. There is no trace of a pecten. The eye is protected externally by the two lids and the nictitating membrane. In some water-tortoises, notably in Chelodina, the lower lid is transparent. Lacrymal and Harderian glands are present.
The SENSE OF HEARING is apparently not very acute, although tortoises and turtles are frightened by noise, and can distinguish sounds; otherwise they would have no voice, which is very tiny and piping in most tortoises during the pairing season. In most water-tortoises the tympanic membrane is thin and quite exposed; in land-tortoises it is often thick and covered by the ordinary skin; lastly, in Chelone the tympanic cavity is filled with a plug of the much-thickened skin, possibly in adaptation to the water-pressure when these creatures dive to considerable depths. The ossicular chain is mostly reduced to a long, bony, columellar rod.
The SENSE OF SMELL is well developed. All Chelonians carefully smell their food, in the air as well as under water. The individual predilection shown by many species for different kinds of animal and vegetable food,–since they are, for instance, able to distinguish between the various sorts of cabbage, cauliflower, sprouts, etc.,–proves that they possess a considerable amount of smell and taste.
Tortoises have a fine sense of touch; even the slightest tap on the shell is noticed, and the skin of the soft parts is extremely sensitive. Tickling of the sides of the tail, or of the hinder surface of a thigh, produces ridiculous scratching actions of the same or of the opposite foot.
The digestive apparatus is simple. Only a few peculiarities need be mentioned. The tongue is mostly broad and soft; it cannot be protruded. The oesophagus of the Chelonidae is covered with many conical projections pointing towards the stomach. The latter is simple, except in Sphargis. The intestine is devoid of a caecum, but the difference between the small intestine and the rectum is very marked and often abrupt. The cloaca is very roomy. It contains the large copulatory organ, which is unpaired, grooved on its dorsal side, and is altogether constructed like that of the Crocodilia. The large bladder opens ventrally into the urodaeum, a recess of the cloaca; near its base open the urinary and genital ducts. Many water-tortoises possess also a pair of lateral thin-walled sacs, the so-called anal sacs, dorso-lateral diverticula of the walls of the urodaeum. These sacs, which have highly vascularised walls, are incessantly filled and emptied with water through the vent, and act as important respiratory organs. When such a water-tortoise, for instance an Emys or a Clemmys, is suddenly taken out of the water, it squirts out a stream of this water, which is not, as is generally supposed, the urine from the bladder.
The mode of respiration is interesting. The lungs are very complicated, highly-developed, spongy structures. They are attached by their whole dorsal surface to the inner lining of the shell. As they cannot expand through their own initiative, and since the shell has made costal and abdominal expansion impossible, the tortoise has to resort to other means of producing the necessary vacuum. This is done partly by the neck and the limbs, which act like pistons in being drawn in and out; partly by the greatly developed hyoidean apparatus, by which, when the neck is stretched out, the throat is alternately inflated and emptied, the air being swallowed, or pumped into the lungs. Additional respiration, besides that of the anal sacs mentioned above, is effected in various aquatic tortoises by slightly vascularised recesses of the pharyngeal region. Most Chelonians can exist for a very long time without breathing; sulky individuals remain for hours or days under water. Cistudo can shut itself up for an equally long time. Nevertheless this and other land-tortoises easily get drowned.
All Chelonians lay white eggs, round or oval, according to their kind, but the shape of the eggs of one set sometimes varies within the greatest limits. The shell varies from a parchment-like, flexible, scarcely calcareous cover to a hard, well-polished case. As a rule the eggs, imbedded in the ground, are hatched after a few months, but in some of the northern kinds, e.g. Emys orbicularis, the hatching is deferred until the next spring, the embryo's development being arrested during the winter. How such eggs, buried a few inches only below the surface, withstand the often very severe North German and Russian winter is a mystery. Whilst the plastron is generally flat, it is more or less concave in the males of many species, notably in Testudo, Cistudo, and Emys.
The general conclusions which can be drawn from the present geographical distribution of the Chelonia are as few and unsatisfactory as those applying to the Crocodilia, since all the main groups of Chelonians, and many more extinct families, occurred together in bygone ages in the same countries, for instance in Europe.