Reptiles

Reptiles are the first entirely terrestrial vertebrate class, totally independent from the aquatic habitat for survival. They have excellent evolutionary innovation to get them rid of water life. Their skin is keratinized and impermeable to water whereas amphibian skin is permeable. Due to keratinized skin, the hitherto cutaneous respiration became impossible and respiration became dependent on internal organs such as airways and lungs.  Snakes and lizards shed their scales as skin cast.

Distribution and sub-groups

Reptiles are found in all continents except Antarctica. There are several sub-groups such as:

• Testudines: This includes some 400 species of turtles, terrapins and tortoises.

• Sphenodontia: This includes only two species of tuatara (found in New Zealand)
• Squamata: This is largest group of reptiles having some 9600 species of lizards and snakes
• Crocodilia: This group has some 25 species of crocodiles, gavials, caimans, and alligators.

Reptiles are both carnivorous and herbivorous. For example, snakes, crocodiles etc. are carnivorous while Iguanas are herbivorous.

Pulmonary Respiration

Since reptiles have no permeable skin, they need to respire using internal organs. Thus, like birds and mammals, reptiles also have pulmonary respiration.

Heart of Reptiles

Like amphibians, the reptile heart has three chambers (two atria and one ventricle). However, their heart is advanced from amphibians because ventricular separation (process of having two ventricles) appears in their heart. Further, Heart is four-chambered in crocodiles.

Greater mobility

Reptiles have larger and more powerful legs. The placement of the reptilian legs beneath the body (instead of at the side as in amphibians) enabled them greater mobility.

External Ear

Reptiles do not have external ear openings. Tympanum represents ear.

Body temperature

Fish, amphibians as well as Reptiles are hetero-thermic animals; which means that they are unable to maintain the body temperature. They are also called Cold blooded animals or poikilothermic or ectothermic animals. This is the reason that

• Reptiles are rarely found in polar regions.
• They need an external heat source to warm their bodies.
• They are more active during the day, a period when they can use the sun’s heat to warm their bodies.

Excretion

Evolution of reptiles happed around water economy. They excrete uric acid which is less toxic than both ammonia and urea and needs least water.

Reproduction

In reptiles, birds and mammals, fertilization is internal by means of copulation between male and females. Reptiles are oviparous and they lay eggs with shell. The embryo develops outside mother’s body. Some reptiles also show ovovivipary in which mother keeps the eggs in its body until it hatches. However, it is different from vivipary as there is no trophic / placental connection between embryo and parent.

Further, the embryonic development is direct in reptiles, which means that there are no tadpoles or larvae.