The class Mammalia includes animals defined by physiological and anatomical traits. These characteristics include mammary glands for nourishing their young, a body covering of hair or fur at some stage of life, and a unique jaw structure that features three distinct bones in the middle ear. Mammals are endothermic, meaning they internally generate and regulate their own body heat, maintaining a relatively constant internal temperature. This ability to self-regulate temperature sets them apart from the other major classes of vertebrates.
The Class Aves
Birds, belonging to the class Aves, share the mammalian trait of endothermy. Their entire body is covered in feathers, which are modifications of the skin and serve as highly effective insulation. The skeletal system of a bird is significantly adapted for flight, featuring many hollow bones reinforced by internal cross-struts, known as pneumatic bones, which reduce overall body weight.
These flying vertebrates possess a fused collarbone (furcula or wishbone) and a large, keeled sternum, which provides an attachment point for powerful flight muscles. Birds lack teeth, instead featuring a lightweight, keratinous beak or bill, the shape of which is highly specialized according to the species’ diet. Their respiratory system is unique, involving a unidirectional flow of air through the lungs with the aid of air sacs, ensuring a highly efficient exchange of oxygen necessary for sustained flight. Reproduction in Aves involves laying hard-shelled, yolky eggs. The forelimbs of birds are modified into wings, and while not all species fly, this anatomical structure is a signature trait of the class.
The Class Reptilia
Reptiles, categorized in the class Reptilia, are distinguished by several characteristics. Unlike mammals and birds, reptiles are ectothermic, meaning they rely on external heat sources like the sun to regulate their body temperature. This reliance dictates behaviors such as basking to warm up.
The skin of a reptile is covered in dry, rough scales or scutes, which are made of keratin and help prevent water loss through evaporation. This scaly skin is generally devoid of glands, contrasting sharply with the glandular, often hair-covered skin of mammals. Reptiles are amniotes, meaning their embryos develop within a fluid-filled membrane; their eggs are typically leathery or hard-shelled and are laid on land, a feature shared with birds. The class includes lizards, snakes, turtles, and crocodiles. Crocodilians are the exception to the standard three-chambered heart found in most reptiles, possessing a four-chambered heart similar to mammals and birds.
The Class Amphibia
The class Amphibia represents vertebrates with a distinctive “dual life” cycle, involving metamorphosis from an aquatic larval form to a terrestrial or semi-terrestrial adult form. Amphibians possess a highly permeable, moist skin that is smooth and largely scaleless, used for cutaneous gas exchange.
This skin structure necessitates that amphibians, including frogs, toads, and salamanders, remain in or near moist environments to prevent desiccation. Like reptiles, amphibians are ectotherms, with their body temperature fluctuating with the surrounding environmental temperature. The larval stage is typically aquatic and breathes using gills, while the adult stage usually develops lungs for air breathing.
Aquatic Vertebrates
Aquatic vertebrates, commonly referred to as fish, are defined by their adaptations to a water-based existence. Fish primarily respire by extracting dissolved oxygen from water using gills. Their locomotion is achieved through fins, which function for propulsion, steering, and balance.
Most fish are ectothermic, with their body temperature varying with the water temperature, although some active swimmers like tuna can maintain a slightly elevated core temperature. The body covering typically consists of overlapping scales for protection and reduced drag, though some species lack scales entirely. Aquatic vertebrates can be broadly categorized into cartilaginous fish, such as sharks and rays, which have skeletons made of cartilage, and bony fish, which possess a calcified internal skeleton and often a swim bladder for buoyancy control.