But today, only a few orders of reptiles remain on this planet and they belong to the class Reptilia and are classified into four orders. They are Crocodilia, which consists of 23 species and include alligators, crocodiles, caimans and gavials; Sphenodontia, with two species of tuatara (a lizard-like reptile); ‘Squamata’, which includes ‘amphisbaenia’ (worm-like lizards), snakes and lizards with around 8,000 species and ‘Testudines’, that consists of 300 species of turtles and tortoises.
Reptiles are vertebrates (with a backbone), which can breathe air and are cold-blooded (cannot regulate body temperature). They are considered as tetrapods (with four legs or leg-like appendages), even though legs are absent in some of these animals, like snakes. Their bodies are covered with scales or scutes and they lay amniotic eggs (embryos develop inside amniotic membranes) on land. They share the characteristics of both amphibians and mammals, but are different and unique. For example, their skin is unique in the sense that it lacks feathers, as in the case of birds; it does not have hair, like that of mammals and it is not moist as in amphibians. One of the unique characteristics of reptiles is their skin, which is tough and scaly. The following are some of the reptile characteristics.
As mentioned earlier, reptiles have a dry skin with scales or scutes. The skin has a few cutaneous glands and high levels of keratin, which prevents water loss through the skin. The scales and scutes, are formed from the epidermis and are also made of keratin, to protect the body.
These animals are considered tetrapods with two sets of paired limbs. Most of these animals have five clawed toes on each limb. These muscular appendages are angled downward to facilitate faster locomotion. In some reptiles, like snakes and worm lizards, the legs are absent, but it is believed that these animals evolve from some tetrapod ancestor.
All reptiles have spinal columns and a strong skeletal system with a rib cage. They have a well-developed brain and a central nervous system. A pelvic region with a minimum of two spinal bones is also seen in most of these animals.
Unlike amphibians, reptiles do not pass through an embryonic stage with gills. These animals breath with well-developed lungs, right from birth. Most of them have two lungs, except some snakes, which possess only a single lung. They don’t possess gills and even cutaneous respiration is very negligible.
All reptiles have three-chambered hearts, except crocodiles, which have four-chambered hearts (2 atria, 2 ventricles), like mammals and birds. The three chambers consist of two atria to receive blood and one partially divided ventricle for pumping blood.
Reptiles have a digestive system, which is similar to other vertebrates, but the gut, the ducts of urinary and sexual organs lead to one posterior chamber, called the ‘cloaca’, which has a muscular opening at the base of the tail. In case of those, who inhabit arid regions, water is further absorbed into the body from the waste in the cloaca and only the remaining waste is excreted.
As far as reproduction is concerned, reptiles are unique, because they are the first animals with amniotic eggs, that can be laid on land and not in water. These amniotic eggs can survive without water, as they have protective shells and membranes, that allow oxygen and other gases to enter. Most of the reptiles lay eggs, but some of them give birth to young ones, by hatching the eggs inside the body of the mother. Reptile characteristics include internal fertilization, as sperm gets deposited into the reproductive tract of the female directly.
The offspring of reptiles resemble the adults at the time of birth itself. There is no metamorphosis, as in the case of amphibians.
Being cold-blooded, their body temperature vary with the surrounding atmosphere. Like in mammals, the body temperature of reptiles is not regulated by internal mechanisms. So they have to maintain the body temperature by basking in the sun. This is the reason behind the presence of many reptiles in arid regions, like deserts. However, they can be seen in different habitats and if the temperature levels are not favorable, some of the reptiles prefer hibernation too.
Physical characteristics of reptiles include their keen sense organs, which help them to find food and escape from predators. Eyes are one of the most important sense organ and in most reptiles, these organs are located at the front of the head to facilitate binocular vision. While most of the different types of lizards can move each eye independently, some of them have a protective cover above the eyes. Most of them don’t have external ears and the eardrum is located near the eyes.
In short, these animals can be seen in diverse habitats, ranging from ponds, seas, lakes, treetops, deserts and mountain ranges, but they are abundant in deserts and arid regions too. The only limitation for them is that they are cold-blooded and cannot generate body heat of their own. This is the reason behind the absence of reptiles in the Antarctic region.
Read more at Buzzle: http://www.buzzle.com/articles/reptiles-characteristics-of-reptiles.html