Reptiles are a class of heterothermic vertebrates, i.e. cold-blooded animals where the body temperature is regulated by the external temperature not having internal regulation mechanisms (as it happens in mammals and birds).
Reptiles are defined as amniotic vertebrates due to the fact that embryo develops in a chamber filled with liquid, theamnios or amniotic sacto protect from dehydration and from external adverse agents (a characteristic that, for example, is not found in amphibians)
They are an evolution of the amphibians and represent the conquest of the terrestrial environment by part of the vertebrates.
HABITAT AND GEOGRAPHICAL DISTRIBUTION
Reptiles are animals widespread in all parts of the world with the exception of the coldest areas of the planet due to the fact that they are unable to regulate their body temperature independently of the environmental temperature in fact, where the winter temperatures drop within certain limits, they go into hibernation while if the temperatures rise beyond certain limits or the climate is excessively dry they go into aestivation.
Reptiles are almost never large animals, not exceeding 10 m in length.
A typical feature of reptiles is the body covered with horny scales which at times form a real exoskeleton (as in crocodiles), which serve to protect them from dehydration and from any mechanical accidents.
The skeleton is mostly ossified (not cartilaginous) and the skull is connected to the vertebral column by means of a single joint as occurs in birds (while in amphibians and mammals there are two).
The limbs in reptiles are usually reduced or absent and when present they are numbered four, each provided with five fingers but there are cases in which they are reduced to two.
They may have strong teeth or be absent as in turtles.
They are equipped with salivary glands which in some ophidians are transformed into veleniferous glands.
Reptiles have very keen eyesight and an equally developed sense of smell.
Most reptiles are oviparous that is to say that fertilization takes place inside the animal's body but the eggs are then laid and complete their development outside the body. However, there are cases of ovoviviparity in which the embryo develops in the egg. still inside the maternal oviduct and the already formed specimen is born directly. There are also cases of viviparity.
Individual bird species information sheets