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Animal Facts


Learn interesting facts about animals, a group of living organisms that includes amphibians, birds, fishes, invertebrates, mammals and reptiles. There are millions of species of animals alive today. The articles listed here provide facts about animals including their characteristics, classification and evolution.

Select from the links below for facts about each animal group:

  1. Basic Animal Facts
  2. Amphibians
  3. Arthropods
  4. Birds
  5. Cnidarians
  1. Fishes
  2. Mammals
  3. Molluscs
  4. Reptiles

Basic Animal Facts

Animals - Animalia

Let's begin with a few basic animal facts. Animals are multicellular organisms that are capable of locomotion and rely on other organisms to obtain their nourishment. There are many different groups of animals, among which the better known groups are amphibians, birds, invertebrates, fishes, mammals, and reptiles.


Amphibians - Amphibia

Amphibians are four-limbed vertebrates that include salamanders, newts, caecilians, frogs, and toads. Early amphibians arose during the Devonian Period, 370 million years ago, and were the first vertebrates to venture out from the water and adapt to life on land. Despite their early colonization of land, most amphibians have never fully severed their ties with aquatic habitats.


Arthropods - Arthropoda

Arthropods are invertebrates that include insects, spiders, crustaceans, scorpions, and centipedes. Arthropods are bilaterally symmetrical and have segmented bodies. Their body is covered with an exoskeleton and many arthropods have compound eyes. Arthropods are a highly successful group of animals—they account for over three quarters of all known living and fossil organisms.


Birds - Aves

Birds are four-limbed vertebrates that have wings, feathers, hollow bones, and other adaptations for an aerial lifestyle. Flight consumes a great deal of energy and consequently these warm-blooded animals have high metabolic rates. Birds evolved from reptiles during the Mesazoic Era about 150 million years ago. Today, an estimated 9000 species of birds inhabit our planet.


Cnidarians - Cnidaria

Cnidarians are a group of invertebrates that includes jellyfish, hydra, sea anemones, corals, and sea pens. Cnidarians have radial or biradial symmetry. The basic body form of cnidarians is simple, consisting of a stomach sac with a single opening. The life cycle of many cnidarians includes a free-swimming medusa and a sessile polyp.



Fishes were among the first vertebrates to evolve. The earliest known fishes were the ostracoderms, a now-extinct group of jawless fishes that appeared in the Cambrian Period. Other early fishes include the conodonts and the agnanthans. Fishes later evolved jaws and diversified into a number of lineages including cartilaginous fishes, ray-finned fishes and lobe-finned fishes.


Mammals - Mammalia

Mammals are four-limbed vertebrates that have mammary glands, hair, a diaphram, a four-chambered heart, and a large cereberal cortex. Seventy million years ago when reptile diversity declined, mammals took over habitats once unavailable to them. Mammals have successfully adapted to and colonized a wide variety of habitats including land, air, and water.


Mollusks - Mollusca

Molluscs are a highly diverse group of invertebrates that include squid, octopuses, cuttlefish, nudibranchs, snails, slugs, limpets, sea hares, mussels, clams, oysters, scallops and many other groups of organisms. There are more than 100,000 species of molluscs making them second in diversity to only the arthropods. The branch of zoology devoted to the study of molluscs is known as malacology.


Reptiles - Reptilia

Reptiles are four-limbed, cold-blooded vertebrates that evolved from amphibians about 340 million years ago. There are two characteristics that distinguished early reptiles from amphibians and enabled them to colonize terrestrial habitats more extensively than their ancestors: scales and the ability to lay hard-shelled amniotic eggs.

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