Animal Facts
By Laura Klappenbach, About.com Guide to Animals / Wildlife
Scientists estimate that there may be between 3 and 30 million species of animals on our planet. The exact number of species on Earth remains unknown, and new species are discovered all the time. Animals inhabit all corners of the globe, from the icy polar regions, to the depths of the ocean, to the tops of mountains. The profiles available at this site will help you become familiar with just a few of the many animal groups with whom we share our planet.
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. All animals have cells that lack rigid cell walls (like those found in plant cells).
- What Makes an Organism an Animal?
- How Many Animal Species Are There?
- Guide to Vertebrates / Invertebrates
Amphibians
Amphibians (Class Amphibia) include animals such as salamanders, newts, caecilians, frogs, and toads. These animals' ancestors were the first to venture out from the water and adapt to life on land. Amphibian larvea are often aquatic and go through a complex metamorphosis process as they grow to adulthood. Amphibians first appeared during the Devonian Period, 370 million years ago.
- Amphibians - Class Amphibia
- What are Caecilians?
- Why are Amphibians in Decline?
- The Life Cycle of a Frog
Arthropods
Arthropods (Phylum Arthropoda) 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 currently known living and fossil organisms.
- Lobster's Unique Sounds Defy Predators
- African Spider Preys on Mosquitos
- The Tangled Web of Spider Classification
Birds
Birds (Class Aves) are unique creatures having 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
Cnidarians (Phylum Cnidaria) are characterized by radial or biradial symmetry. Members of the Phylum Cnidaria includes jellyfish, hydra, sea anemones, corals, and sea pens. The basic body form of cnidarians is simple, consisting of a stomach sac with one opening. The life cycle of many cnidarians includes a free-swimming medusa and a sessile polyp.
Insects
Insects (Class Insecta) are a highly successful group of animals. With somewhere between 750,000 and 30 million species of insects alive today, insects account for more species of animals than any other class of animals. The body of an insect is made up of a head, thorax, abdomen, a pair of compound eyes, a pair of antennae, and a set of complex mouthparts.
- Insects - Class Insecta
- The Cicadas are Coming
- New Order of Insects Identified
- 17-Year Cicadas: From Nymph to Adult
Mammals
Mammals (Class Mammalia) are characterized by mammary glands, hair, a diaphram, a four-chambered heart, and a large cereberal cortex. 70 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.
- Mammals - Class Mammalia
- An Introduction to Mammals
- Mammal Evolution and Radiation
- Nine Characteristics of Mammals
Molluscs
Molluscs (Phylum Mollusca) are a highly diverse group of animals that include cephalopods (squid, octopuses, cuttlefish), gastropods (nudibranchs, snails, slugs, limpets, sea hares), bivalves (mussels, clams, oysters, scallops) and many other groups of organisms. There are an estimated total of more than 250,000 species of molluscs. The branch of zoology known as 'malacology' is the study of molluscs.
- Mollusks - Phylum Mollusca
- Gastropods, Slugs and Snails - Class Gastropoda
- A Natural History of Terrestrial Snails
- California Sea Hare - Aplysia californica
- Ten Facts About Octopi
Ray-Finned Fishes
Ray-Finned Fishes (Class Actinopterygii) are a diverse group of aquatic vertebrates—over half of all living vertebrate species (aquatic and terrestrial) belong to this class. The first bony fishes apeared during the Devonian Period and underwent significant expansion and diversification during the Carboniferous Period. During the Late Triassic a group of ray-finned fishes called 'teleosts' first appeared.
- Ray-Finned Fishes - Class Actinopterygii
- Cichlids - Family Cichldae
- Firefish - Pterois volitans
- Ten Facts About Fishes
- Fish Fall Prey to Invading Lionfish
Reptiles
Reptiles (Class Reptilia) are 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. Reptiles gave rise to the birds and the mammals.
