Page:The American Cyclopædia (1879) Volume XV.djvu/695

 TESTUDINATA 665 or dermal skeleton, divided into many pieces united by suture; in the marine turtles this dermal skeleton is imperfect (especially be- low), less developed in the trionyx, and least of all in the trunk turtle (sphargis). The epidermic plates in the tortoise-shell turtle grow only on the anterior edge, the older parts moving backward, much as in the human nail ; but in the land tortoises they increase below and on all sides, in concentric rings, like the annual growths of a tree ; there is every intermediate stage between these types ; a moulting of the epidermis takes place in all chelonians, scale by scale. In all except the imbricated turtle the colors are in the lowest layers of the epidermis ; in this they exist in the external dry horny layers, displaying the beautiful and permanent hues of tortoise shell ; in the corium or true skin is deposited the phosphate of lime of the dermal skeleton. The skull is solid and compact, and the facial bones are immovably fixed to the cranium ; the lower jaw consists of a firm bony arch ; the occipital bone strikingly resembles a vertebra; the pari- etals principally enclose the brain; there are two pairs of f rentals, and the nasals are almost always wanting. The cervical vertebrae are nine, if the odontoid process be considered dis- tinct, and have no transverse processes ; some have a concave-convex articulation, others a convex-concave, one a biconcave (toward the lower part of the series), and one a biconvex (in the middle), giving considerable freedom of motion in certain directions without the flexi- bility of the bird's neck. The dorsal vertebrae are 11, of which the first is movable, the rest united into a firm arch by the continuous growth of the spinous processes ; the ribs ex- tend from between the vertebras, being strong- est where the dermal skeleton is least devel- oped, as in trionyx, sphargis, &c. ; the sternum consists of four pairs and one odd bone, vary- ing much in size and connection, united to the ribs by a bony bridge, the marginal plates be*- ing dermal bones ; the caudal vertebrae are very movable, convex behind, concave before, and without spinous processes. The scapular and pelvic arches are withdrawn under the bony roof of the body ; the bones of the shoulder are long, straight, and narrow, the scapula and acromion united at right angles, the coracoid running backward among the muscles, and the three united to form the glenoid cavity; the humerns is short, crooked, and turned inward ; the forearm and hand have their transverse di- ameter vertical, the ulna overlying the radius, so that the limb may be drawn back under the carapace by the bending of all the joints in the plane of the scapula; the form of the hand varies in the different families, according as it is used for terrestrial or aquatic locomotion. The pelvic arch is formed by three permanently distinct bones, which meet in the cotyloid cav- ity ; the bones of the hind legs are like those of the anterior, but the femur is straighter than the humerus ; there are great differences in the relative size of the two pairs of legs in the two suborders. The cervical muscles are largely developed ; the muscles of the limbs are much like those of mammals. The cere- bral hemispheres are hollow and larger in pro- portion than in other reptiles, with a generally smooth surface. There is a tympanic cavity and membrane, the former divided into two parts by a bony partition ; the eyes are larger and more movable than in the lower reptiles, similar to those of birds in the lids, nictitating membrane, osseous framework of cornea, and round pupil; a lachrymal gland is present. Hearing and vision are acute, but smell is dull, the nostrils being used chiefly for their slow respiration; they chew their food, and the tongue is broad, thick, and fleshy, with an acute sense of touch ; the tongue is of use also in the respiratory process, as they swallow air into the lungs. The upper jaw always shuts over the lower, and both are covered with a peculiar horny sheath ; the intestines, as in the higher classes, are longest in the herbivorous families, but the proportions of the different parts vary much without any special reference to the food; the liver and gall bladder are large ; spleen and pancreas always present, the former solid and generally attached to the lat- ter, and this to the duodenum ; the pancreas is lobular and irregular, and much the largest in the carnivorous feeders ; digestion is per- formed very slowly, and hunger can be endured a long time. Respiration is effected by swal- lowing air, on account of the immobility of the thoracic cavity, assisted, according to Agassiz, by the diaphragm, which is well developed in the order, and by the scapular and pelvic mus- cles ; the lungs are voluminous, most so in the land tortoises ; the trionyx can remain half an hour or more under water, aeration of the blood in this and other aquatic species being doubtless partly effected, as in frogs, through the naked skin ; many species have the power of emitting vocal sounds, independent of the sharp hiss which they all produce ; respiration is reduced or entirely suspended in the hibernating spe- cies, according to the degree of this state. The heart is just above the liver, between its halves ; the ventricle is single, divided into two cavities by an imperfect partition, and gives rise both to the two aortae and the pulmonary artery; it beats about ten times a minute ; the lym- phatic system is greatly developed, two hearts near the base of the tail sending the lymph over the body. The kidneys are comparatively small, flattened and lobed, in the pelvic cavity, outside the peritoneum ; the ureters short, and bladder large ; the ovaries are much like those of birds, and the number of eggs matured in a year varies in different members of the order ; the cloaca is very large in both sexes. All are oviparous, and the eggs are spherical, covered with a hard shell, and laid in moist or dry ground or hot sand, the number varying from four or five in the land tortoises to more than 100 in the marine turtles; the young, which