Animal Kingdom Classification


Mammals are vertebrates that have hair, and the females have mammary glands that produce milk for their young. These glands only become active after bearing young, and are a product of epithelial cells underneath the skin. Mammals can be categorized into three groups. Monotremes lay eggs. Marsupials have a pouch to nurse their young, which are born in an undeveloped state. Placentals' young are nourished and develop in the uterus. Mammals are warm-blooded or endothermic, meaning they have a constant warm body temperature that's internally regulated.

Mammal, Mountain Goat, Mom and Baby, Oreamnos americanus
Mammal, Mountain Goat, Mom and Baby,
Oreamnos americanus


Reptiles are cold-blooded vertebrates that breathe with lungs and have skin covered in scales. Most reptiles are amniotic egg layers, though some give birth to their young. The classification of reptiles includes alligators and crocodiles, turtles and tortoises, lizards and snakes, and the endangered tuatara of New Zealand.

Reptile, Western, Prairie Rattlesnake, Crotalus viridis viridis
Reptile, Western, Prairie Rattlesnake,
Crotalus viridis viridis


Amphibia are a class of vertebrates that includes toads, frogs, newts and salamanders. Amphibians generally spend part of their lives in water and part on dry land. They develop in two stages where the larval state is generally aquatic, when they may breathe with gills. Then they go through a metamorphosis, which adapts them to life on dry land. Their skin has no scales but is soft and moist. They lay their eggs in moist environments due to the lack of an amniotic membrane. Amphibians are cold-blooded, or ectothermic, animals, which have a variable body temperature that's regulated by their external environment. Because of this many amphibians hibernate during the winter.

Amphibian, Frog, Bullfrog, Rana catesbeiana
Amphibian, Frog, Bullfrog,
Rana catesbeiana


Birds, the largest number of tetrapod vertebrates, are warm-blooded egg layers and are unique in their covering of feathers. While most birds can fly, some, such as the ostrich and penguin are flightless, but all have forelimbs that have evolved into wings. Their feathers developed from scales and provide a flight surface, while scales on their legs can still be seen today. While most birds are active during the day, some, such as the owl, are active at night. Birds are very popular animals as can be seen by the hobbies of birding and birdwatching.

Bird, Common Goldeneye Duck, Male, Bucephala clangula
Bird, Common Goldeneye Duck, Male,
Bucephala clangula


Most fish are cold-blooded, entirely aquatic vertebrates that have paired fins, torpedo-like bodies, and lay eggs. Many breathe by means of gills, but a few breath atmospheric air via lung-like organs or other adaptations of their bodies. A few saltwater fish, such as tuna, swordfish, and some sharks, are able to maintain body temperatures above that of the surrounding water. Many fish have highly developed senses of sight and smell, as well as the ability to detect movement and vibration in the water with their lateral line organ. Scales are commonly found on bony fish, while some fish, such as catfish and eels, have no scales. Fish are very popular animals, as a commercial food source, in sport fishing, and as entertainment in public aquariums and private fish tanks.

Fish, Lingcod, Ophiodon elongatus
Fish, Lingcod,
Ophiodon elongatus


Insects are animals that have exoskeletons. An external skeleton of chitin supports their body structure, which is comprised of three parts, head, thorax and abdomen. They have six legs that occur in pairs and most have wings. Insects belong to the subphylum Hexapoda, which means six legs in Greek. Insects are the most numerous and prolific group in the animal kingdom. Their heads have compound eyes, antennae and mouthparts, which vary in design from wasp and beetle chewing to piercing as in aphids and mosquitoes. Insects are sensitive to their environment, from their compound eyes, to tiny hairs on their antennae and bodies, which are sensitive to touch, taste and smell. Insects generally go through a transformation from young to adult, beginning with either larval or nymph states. Larvae typically shed exoskeletons in molts, or go through a pupa state, before transforming into their adult form.

Insect, Carolina, Grasshopper, Locust, Dissosteira carolina
Insect, Carolina, Grasshopper, Locust,
Dissosteira carolina


Arachnids are insect-like arthropods with bodies comprised of two parts, the abdomen and the cephalothorax, which contains the head and thorax. The word Arachnid is derived from a Greek word meaning spider. In comparison to insects, their eyes are simple instead of compound. They lack wings and antennae, and have four pairs of legs instead of the six found in insects. Arachnida as a group include scorpions, spiders, ticks and mites. Most arachnids are carnivorous, but some are parasitic. Others are herbivorous and destroy commercial crops. Some arachnids have a bite or sting that's poisonous to humans. Most spiders can inject venom, though only a minority, for example, the black widow, are harmful to humans.

Arachnid, Black Widow Spider, Latrodectus mactans
Arachnid, Black Widow Spider,
Latrodectus mactans

Andrew Williams /

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