The size of an adult turtle varies enormously depending on the species. The species still in existence range from approximately 10 cm long for Sternotherus odoratus (freshwater turtle) (Fig. 1) to 200 cm long for Dermochelys coriacea (sea turtle); although this latter may seem huge, its dimensions are nothing in comparison with those of some fossils found by palaeontologists, which have given rise to estimates of extinct species about 4 metres long.
The measurements of a turtle are taken linearly on the perpendicular axes of the shell. Turtles are very long-living reptiles whose life expectancy varies considerably from one species to another: as a general rule, terrestrial turtles live longer than aquatic ones and it is thought that Emydidae easily pass 30 years, while Italian land-dwelling turtles have a life expectancy well beyond 50 years. There are documented cases of turtles having exceeded 200 years of life!
The shell is the supporting structure of turtles. It is an armature that protects the body of the animal and enables attachment of the muscles of the limbs. It also functions as a reservoir of calcium which acts both as a buffer system to cope with the acidosis that develops during the period of hibernation and as a valuable source of the mineral for the production of the shells of the eggs. The shell is formed of a convex dorsal part, called the carapace, and a flat or slightly concave ventral part, called the plastron, which are connected laterally to each other by a bony bridge. Both the carapace and the plastron are formed of numerous bony plates (Figs. 2a, 2b and 2c) joined to each other by suture lines. Both structures are covered by “scutes”, horny scales of dermal and epidermal origin which cover the bony plates but without following their lines of connection (Figs. 3a and 3b). The shell is quite soft and pliable in the first year of life, but in following years it gradually hardens until reaching a consistency that is sufficient to protect the body of the reptile.
There are eight cervical vertebrae which give the neck great mobility, while the ten thoracic vertebrae are fused to the carapace. These are followed by two sacral vertebrae and a variable number of coccygeal vertebrae which can reach as many as 30. The limbs are short and stubby and finish with five digits.
The turtle’s beak does not have teeth but both the mandible and the maxilla have very sharp, cutting edges that slice food into small pieces before ingestion, which in these species occurs directly without mastication. The tongue within the buccal cavity is fleshy and poorly mobile, but covered with taste buds. The mouth, which lacks a soft palate, communicates with the nasal cavities, which are separated from the buccal cavity only by the hard palate. The eyes, situated laterally, have two external eyelids as well as the internal nictitating membrane. The skin is fairly inelastic and may be smooth, in aquatic species, or covered in hard scales, in the land-dwelling species. The permeability of the skin varies according to the environment in which the animal lives (being greater in the aquatic species and less in those living in a desert environment). Unlike other reptiles, turtles do not undergo a true moult, but change their skin gradually; the scutes are also replaced continuously.
The internal cavity, called the coelom, is not divided into thoracic and abdominal cavities, but consists of a single chamber that houses all the internal organs. The lungs are located dorsally to the other organs and separated from these by a horizontal septum. The heart has three chambers (two atria and a single ventricle) located on the median line just above the plastron. One peculiarity of the chelonian circulation is the portal-renal system, which enables blood from the posterior part of the body to be directed to the liver or diverted to the kidneys. The trachea of testudines is fairly short and bifurcates before entering the thorax, while it is longer in other species. The lungs, as already mentioned are situated dorsally, immediately under the carapace to which they adhere, and do not, therefore, collapse in the case of perforation of the shell. The liver is divided into two lobes which surround the heart and together with this occupy the cranial part of the coelomic cavity. The liver is connected to the stomach by a ligament on the left lobe and, on the right, to the duodenum and pancreas. The caecum is small. The digestion in the herbivorous species is very slow and can last even several weeks, being affected, above all, by the external temperature. The digestive tract terminates in the cloaca which is divided into the urodaeum, coprodaeum and proctodaeum.
The kidneys, located dorsally and caudally outside the coelom, do not have loops of Henle and so urine is concentrated mainly in the bladder. The kidneys communicate with the urodaeum. The testicles are also located dorsally and caudally, close to the kidneys. The penis is in the tail and is extroverted under the effect of the corpora cavernosa. The females have two ovaries situated in the coelomic cavity in the posterior half of the body. The eggs are stored in the oviduct and the vagina opens into the cloaca (Fig. 4).