Chapter 15 Reptiles and fish
The class Reptilia includes about 6500 species, all of which breed on land. The class is divided into four orders of which only two are significant as far as exotic pets are concerned. The four orders are:
These animals share many anatomical and physiological features so general reptilian anatomy and physiology will be discussed first, and any specific adaptations will be mentioned in the subsequent sections on lizards, snakes and the shelled reptiles.
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.
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.
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.
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.
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.
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 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.
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.
Tortoises, turtles and terrapins are members of the order Chelonia (N.B. in the USA all shelled reptiles are referred to as turtles). They are characterised by a hard outer shell consisting of a domed upper part called the carapace and a flatter ventral part called the plastron (Fig. 15.1). 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 the most adjacent part of the body. The scutes grow from the 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 to estimate age.
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.
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.
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).
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.
Stomatitis or ‘mouth rot’ is an infection of the oral cavity and is a very common condition of chelonians and snakes. This condition is often triggered by factors such as stress, trauma, malnourishment and poor husbandry. It is commonly seen in chelonians following hibernation and is often the cause of post-hibernation anorexia syndrome.
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.
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.