Reptiles are popular animals that can be easily identified by people due to their typical characteristics. For instance, snakes have always made human beings joining opposite sides of the discussion on whether they like them or not. To begin with, reptiles are a group of slithering animals that have a cold blood, scales (rigid plates) on the skin, and lay eggs for reproduction purposes. Tetrapods (birds, mammals, reptiles, and amphibians) belong to sub-phylum Vertebrates, which ultimately embedded into the phylum Chordata. As Kemp (2019) revealed, the Reptilia class comprises approximately 10,000 different species divided into four living clades present in all continents of the Earth except Antarctica. This Class of Vertebrates include Squamata (snakes and lizards), Testudines (turtles), Sphenodontia (tuataras), and Crocodilia (alligators and crocodiles).
Although there is a variety of body forms and behavior within the Class, there are fundamental characteristics, which are similar for all species. Reptiles have a specific body structure that makes them unique compared to other living creatures. Their body is divided into head, trunk, tail, and two pairs of pentadactyl limbs. The limbs are located on the trunk’s sides (snakes are an exception) and are pressed to the ground, making them move close to the surface. That is why the Class Reptilia received its name because from late Latin this word means “creeping.”
In general, a reptile’s skin is dry and has scales or scutes which protect the animal from moisture evaporation and desiccation. As Kemp (2019) explains, their hearts are partially divided (three chambers); they have slow metabolism generating low heat to keep the stable body temperature resulting in a cold blood. For that reason, reptiles change their location throughout the day regulating their body temperature using external factors. Reptiles use lungs for breathing, and their circulation system is more efficient than that of amphibians. They usually possess two compact kidneys that able to produce urine in semi-solid form.
Moreover, fertilization of reptile eggs occurs inside the female’s body involving the male’s sperm. A leathery membrane protects the eggs, which have water and food reserves. Such kind of egg is called amniotic because it is laid on a dry surface instead of water (Kemp, 2019). An embryo develops into a miniature adult and then hatches. Most reptiles lay up to 16 large eggs covered with leathery or hard calcareous shells. Reptile eggs are usually laid in the soil or rock cracks and under the bark of trees (in the case of lizards). In most species, the breeding period is associated with behavior change: fierce fights between males and females’ courtship by the males. What is more, reptiles have a specific skull, a typical cloaca, and have 12 pairs of cranial nerves.
Now it is time to find out why reptiles belong to the phylum Chordata. The most prominent and essential feature is the availability of a backbone. Another critical feature embedded in classification is a dorsal nerve cord. Reptiles have both the brain and spinal cord that is typical for chordates. The sub-phylum Vertebrates also include a range of standard features: the vertebral column (spinal column), a post-anal tail, formed endoskeleton, bilaterally symmetrical bodies, and have more developed organs compared with invertebrates (Kemp, 2019). All in all, reptiles are a large class of animals consisting of thousands of species and have common features, including cold blood, dry skin, and eggs for reproduction.
Reference
Kemp, T. (2019). Reptiles: A very short introduction. Oxford University Press.