General anatomy
Skeletal system
Reptiles are vertebrates and have an internal bony skeleton which, to some extent, shares the basic skeletal plan exhibited by members of the class Mammalia. However, there are distinctive m(ISA all shelled reptiles are referred to as turtles).
They are characterised by a hard outer shell consisting of a domed upper part called lhe Cftnipace and a Ilalter ventral part called the plastron (Fig. 1 5.1). The shell forms a bony 'box' that protects the soft internal parts of the body. The shell is covered with horny plates or scutes. which are named according to lhe mosl adjacent pari of the body. The scules grow from lhe outside so that an annual ring develops along the periphery of each one making the overall shell larger. In some species these ‘growth rings’ can be used Io estimate age.Anatomical features
Skeleton
Chelonians are vertebrates and their skeleton resembles that of other vrertebrates. However, the pectoral and pelvic girdles are within lhe rib cage and are orientated vertically to buttress the shell (Fig. I 5.1). The ten vertebrae form part of lhe under surface of the carapace.
Cardiovascular system
Chelonians possess the normal reptilian three-chambered heart and renal portal system. 'Γhe outer shell of chelonians makes auscultation of the heart difficult, but may be aided by putting a damp towel around the shell.
Respiratory system
The rigid outer shell of chelonians prevents the body wall from expanding during breathing. Respiration is accomplished with the aid of limb and head movements. which move in and out and alter lhe internal pressure in the body cavity. Chelonians breathe through their external nares or nostrils, so mouth breathing may indicate a respiratory problem. The glottis lies at the base of the tongue, and the trachea is short which allows lhe tortoise to breathe when the neck is withdrawn.
The lungs are positioned dorsally. below the carapace, and aid buoyancy in aquatic species (Fig. 1 5.1).Digestive system
Chelonians do not have teeth and depend on their horny beak to cut off pieces of food. They have large. Ileshy tongues that cannot protrude from the mouth. The oesophagus runs down lhe left side of lhe neck and joins the stomach, which lies transversely across the body (Fig. 1 5.2). The small intestine is relatively short (compared to mammals), and the colon ends in the < ∕scales of the integument show differences according to the region of the body. In the dorsal and lateral parts of the body the scales are small and on the ventral surface they are larger and thicker. 'Γhe tough, smooth skin of the snake does not grow with the snake but is shed periodically, usually in one piece to reveal a new skin underneath (ecιlι∣sis). At the lime of shedding the snake skin may have a dull appearance due to the lifting of the old outer layer of skin. Snakes do not have moveable eyelids, as are seen in mammals, but the upper and lower eyelids are fused together to form a transparent spectacle over the cornea. During ecdysis the spectacle is also shed, just before shedding the snake may be withdrawn and anorexic and handling should be kept to a minimum.
Special senses
The eyes are protected by the spectacle lying over the surface of the cornea. Tear-Iike secretions are produced between the cornea and spectacle. I he ear of the snake has no tympanic membrane and no middle ear cavity. Some species of snake, e.g. members of the family Boiilae - the boas - have heat sensitive pits on the upper jaw which are used to detect their prey. These are so sensitive that they can pick up heat changes of ().()01 oC.
Cardiovascular system
Snakes possess the typical reptilian heart, and have both renal and hepatic portal circulations. The heart is generally located at about one third of the length of the bodv.
Respiratory system
In most snakes the left hint) is greatly reduced in size or even absent (Eig 1 5.1 1). Only the anterior part of the lung is functional for gaseous exchange. The posterior part is avascular and functions as an air sac that may act as a reserve during periods of apnoea.