Skeletal system

Reptiles are vertebrates and have an internal bony skeleton that, to some extent, shares the basic skeletal plan exhibited by members of the class Mammalia. However, there are distinctive modifications in the skeleton of the snakes, tortoises and turtles that will be discussed later.

Cardiovascular system

The heart has three chambers rather than four. There is a right and left atrium but only one ventricle. The ventricle is functionally, but not anatomically, divided into three subchambers and receives blood from both the right and left atria. Deoxygenated blood from the right atrium is directed towards the pulmonary artery, but the oxygenated blood returning from the lungs to the ventricle may pass either to the aortic arches or to the pulmonary circulation again.

A significant feature of a reptile’s peripheral circulation is the renal portal system, which transports blood from the hindlimbs and tail directly to the kidneys. This has clinical implications when injecting into the caudal half of the body, as some of the drug may be excreted in the urine before reaching the systemic circulation.

Respiratory system

Gaseous exchange occurs in the same way as it does in mammals but the most significant difference in the anatomy of the respiratory system of reptiles compared to that of mammals is that reptiles lack a diaphragm. As in the bird, the body cavity is not divided into two. Respiratory infections are common in reptiles but because they lack a diaphragm they lack an active, expulsive cough reflex and these infections can be severe or even fatal.

Digestive system

The more specific features of the digestive system of lizards, snakes and chelonians are covered separately. However, in general, the digestive system terminates in a common exit – the cloaca, consisting of three parts: the coprodeum collects the faeces, the urodeum collects urinary waste and the proctodeum is the final chamber that acts as a collecting area prior to the elimination of the waste.

Urinary system

The paired kidneys consist of nephrons without loops of Henle. This means that they are unable to produce concentrated urine. Chelonians have large bladders, which may occupy up to 25% of their body weight. Not all lizards have bladders, although a bladder is present in the green iguana (Iguana iguana). Snakes do not have bladders. The urine may change within the bladder so urinalysis in reptiles may not be an indication of kidney function, as it is in mammals.

Reproduction

Reptiles are oviparous, i.e. they lay eggs. The yolk of the egg provides the nourishment for the developing young, in contrast to mammals where the young are nourished directly by the mother via the placenta. Some reptiles are ovoviviparous or ‘live-bearing’ – they retain the developing young within the egg, which remains in the oviducts, and appear to give birth to live individuals. However, the nutrients are still obtained from the yolk of the egg inside which the young develop.

The integument

The skin of reptiles is thick and keratinised and is protected by scales or horny plates. Reptiles grow by a process known as ecdysis, during which they shed or slough the old skin. Beneath this is a new layer, which, to start with, is quite soft and easily damaged. Ecdysis varies with species and may be partial shedding, as seen in lizards, or entire, as seen in snakes.

Thermoregulation

Reptiles are ectothermic, i.e. they are unable to regulate their internal temperature and are dependent on the external environment to raise the body temperature and increase their metabolic rate. To do this they employ a number of behavioural patterns, e.g. basking in sunlight or spreading themselves as flat as possible in order to increase the surface area exposed to the sun. Each species has a preferred body temperature (PBT). This is the body temperature at which the reptile functions most efficiently. Below the PBT digestion is impaired and the immune system does not function so that reptiles kept at low temperatures are more likely to become ill.

Skeleton

Chelonians are vertebrates and their skeleton resembles that of other vertebrates. However, the pectoral and pelvic girdles are within the rib cage and are orientated vertically to buttress the shell (Fig. 15.1). The ten vertebrae form part of the under surface of the carapace.

Cardiovascular system

Chelonians possess the normal reptilian three-chambered heart and renal portal system. The outer shell of chelonians makes auscultation of the heart difficult, but it 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 the 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 the tortoise to breathe when the neck is withdrawn. The lungs are positioned dorsally, below the carapace, and aid buoyancy in aquatic species (Fig. 15.1).

Digestive system

Chelonians do not have teeth and depend on their horny beak to cut off pieces of food. They have large, fleshy tongues that cannot protrude from the mouth. The oesophagus runs down the left side of the neck and joins the stomach, which lies transversely across the body (Fig. 15.2). The small intestine is relatively short (compared to mammals) and the colon ends in the cloaca, which is the common chamber into which the urogenital and digestive systems empty.

Fig. 15.2 Internal anatomy of the tortoise.

Urogenital system

In chelonians the ureters conduct relatively unconcentrated urine from the kidneys into a thin-walled urinary bladder (Fig. 15.2). Male chelonians possess a single, large penis that protrudes from the floor of the cloaca.

Sexual differentiation

Most species of Chelonia are sexually dimorphic: there are visible differences in external features such as colouration, tail length, shell size and shape. Features include:

Fig. 15.3 Determining the sex of chelonians. A Female. B Male. The tail of the male is longer, the plastron is concave and the cloacal opening is further down on the tail.

Skeleton

The skeleton of the lizard follows the basic vertebrate plan, but there is no sternum (Fig. 15.4). Most lizards have four legs and most species take their weight on all their legs, however some species, e.g. the basilisk, can run on two legs. The anatomy of the limbs indicates the mode of locomotion.

Fig. 15.4 Skeleton of a lizard.