Animals (Metazoa) are a group of living organisms that includes more than one million identified species and many millions more that have yet to be named. Scientists estimate that the number of all animal species—those that have been named and those that have yet to be discovered—is between 3 and 30 million species.
Animals are divided into more than thirty groups (the number of groups varies based on differing opinions and the latest phylogenetic research). For the purposes of this site, we focus on six of the most familiar groups—amphibians, birds, fishes, invertebrates, mammals and reptiles. After gaining an understanding of those groups, you can explore the other lesser-known groups of animals as well.
Animals are one of several major groups of living organisms. In addition to animals, other groups of organisms include plants, fungi, protists, bacteria, and archaea. To understand what animals are, it helps to be able to articulate what animals are not—in other words, it helps to be aware of what organisms belong to the other major (non-animal) groups and are therefore are not animals.
The following is a list of organisms that are not animals:
- Plants - green algae, mosses, ferns, confiers, cycads, ginkos, and flowering plants
- Fungi - yeasts, molds, and mushrooms
- Protists - red algae, ciliates, and various unicellular microorganisms
- Bacteria - tiny prokaryotic microorganisms
- Archaea - single-celled microorganisms
If you're talking about an organism that belongs to one of the groups listed above, then you are talking about an organism that is not an animal.
So, now that we've listed a few things that animals are not, let's take a look at what animals are and explore some of the characteristics that distinguish them from organisms such as plants, fungi, protists, bacteria and archaea.
Animals are a diverse group of organisms that include many subgroups such as arthropods, chordates, cnidarians, echinoderms, molluscs, and sponges. Animals also include a vast array of lesser-known creatures such as flatworms, rotifers, placazoans, lamp shells, and waterbears. These high-level animal groups may sound rather strange to anyone who has not taken a course in zoology, but the animals that we are most familiar with belong to these broad groups. For example, insects, crustaceans, arachnids, and horseshoe crabs are all members of the arthropods. Amphibians, birds, reptiles, mammals, and fishes are all members of the chordates. Jellyfish, corals, and anemones are all members of the cnidarians.
The vast diversity of organisms that are classified as animals makes it difficult to draw generalizations that describe all members of the group. But there are several common characteristics animals share that describe most members of the group. These common characteristics include multi-cellularity, specilization of tissues, movement, heterotrophy, and sexual reproduction.
Animals are multi-cellular organisms, which means their body consist of more than one cell. Like all multi-cellular organisms (animals are not the only multi-cellular organisms, plants, and fungi are also multi-cellular) animals are also eukaryotes. Eukaryotes have cells contain a nucleus and other structures called organelles that are enclosed within membranes. With the exception of the sponges, animals have bodies that are differentiated into tissues that serve a specific function. These tissues are, in turn, organized into organ systems. Animals lack the rigid cell walls that are characteristic of plants.
Animals are also motile (they are capable of movement). The body of most animals is arranged such that the head points in the direction they move while the rest of the body follows behind. Of course, the great variety of animal body plans means that there are exceptions and variations to this rule.
Animals are heterotrophs, meaning they rely on consuming other organisms to obtain their nourishment. Most animals reproduce sexually by means of differentiated eggs and sperm. Additionally, most animals are diploid (the cells of adults contain two copies of their genetic material). Animals go through different stages of development as they mature (these developmental stages are called the zygote, blastual, gastrula, and blastula).
Animals range in size from microscopic animals known as plankton to the massive blue whale. They inhabit virtually every habitat on the planet—from the poles to the tropics and from the tops of mountain tops to the deep, dark waters of the open ocean.
Animals are thought to have evolved from flagellate protozoa, and the oldest animal fossils date back 600 million years, to the latter part of the Precambrian. It was during the Cambrian period (about 570 million years ago), that most major groups of animals evolved.
The key characteristics of animals include:
- eukaryotic cells
- sexual reproduction
- specialization of tissues
Some of the better known groups of animals include:
- Arthropods (Arthropoda) - There are more than one million known arthropods species and many millions more that have yet to be named. Scientists estimate there may be as many as 30 million species of arthopods (most of which are insects). Members of this group include centipedes, millipedes, spiders, mites, horseshoe crabs, scorpions, insects, and crustaceans. Arthropods are bilaterally symmetrical and have a segmented body, an exoskeleton, jointed appendages, and numerous pairs of legs and specialized limbs.
- Chordates (Chordata) - There are about 75,000 species of chordates alive today. Members of this group include vertebrates, tunicates, and cephalochordates. Chordates have a notochord that is present during some or all of their life cycle.
- Cnidarians (Cnidaria) - There are about 9,000 species of cnidarians alive today. Members of this group include jellyfish, corals, sea anemones, and hydras. Cnidarians are radially symmetrical and have a gastrovascular cavity with a single opening that is surrounded by tentacles.
- Echinodermata (Echinoderms) - There are about 6,000 species of echinoderms alive today. Members of this group include star fish, sea lilies, feather stars, brittle stars, sea cucumbers, and sea urchins. Echinoderms are pentaradially symmetrical and have an endoskeleton composed of calcareous ossicles.
- Molluscs (Mollusca) - There are about 100,000 species of molluscs alive today. Members of this group include bivalves, gastropods, cephalopods, tusk shells, and several other groups. Molluscs have soft bodies that consist of three basic parts: a foot, a visceral mass, and a mantle.
- Segmented Worms (Annelida) - There are about 12,000 species of segmented worms alive today. Members of this group include earthworms, ragworms, and leeches. Segmented worms are bilaterally symmetrical and their body consists of a head region, a tail region, and a middle region of numerous repeated segments
- Sponges (Porifera) - There are about 10,000 species of sponges alive today. Members of this group include calcarious sponges, demosponges, and glass sponges. Sponges are primitive multi-cellular animals that have no digestive system, no circulatory system, and no nervous system.
Find out more: The Basic Animal Groups
Some of the less well-known animal groups include:
- Arrow worms (Chaetognatha) - There are about 120 species of arrow worms alive today. Members of this group are predatory marine worms, some of which attach themselves to surfaces such as rocks. Arrow worms are present in all marine waters including shallow coastal waters to the deep sea. They are found in all climate regions from the tropics to the polar regions.
- Bryozoans (Bryozoa) - There are about 5,000 species of bryozoans alive today. Members of this group are tiny aquatic invertebrates that filter food particles from the water using fine, feathery tentacles.
- Comb jellies (Ctenophora) - There are about 80 species of comb jellies alive today. Members of this group have combs—groups of cilia used for swimming. Most comb jellies are predators that feed on plankton.
- Cycliophorans (Cycliophora) - There are two known species of cycliophorans alive today. The group was first described in 1995 when scientists discovered the species Symbion pandora, more commonly known as the lobster-lip parasite, which lives on the mouth parts of Norwegian lobsters. Cycliophorans have a body that is divided into a mouth-like structure called a buccal funnel, an oval mid-section, and a stalk that ends in an adhesive disc that clasps onto the setae of the loster's mouth parts.
- Flatworms (Platyhelminthes) - There are about 20,000 species of flatworms alive today. Members of this group include planarians, tapeworms, and flukes. Flatworms are soft-bodied invertebrates that have no body cavity, no circulatory system, and no respiratory system. Oxygen and nutrients must pass through their body wall by means of diffusion. This limits their body structure and is why these organisms are flat.
- Gastrotrichs (Gastrotricha) - There are about 500 species of gastrotrichs alive today. Most members of this group are freshwater species, although there are also a small number of marine and terrestrial species. Gastrotrichs are microscopic animals with transparent bodies and cilia on their belly.
- Gordian worms (Nematomorpha) - There are about 325 species of gordian worms alive today. Members of this group are, during the larval stage, parasitoid animals whose hosts include beetles, cockroaches, and crustaceans. As adults, gordian worms do not require a host to survive.
- Hemichordates (Hemichordata) - There are about 92 species of hemichordates alive today. Members of this group include acorn worms and pterobranchs. Hemichordates are worm-like animals, some of which live in tubular structures (also known as a coenecium).
- Horseshoe worms (Phoronida) - There are about 14 species of horseshoe worms alive today. Members of this group are marine filter-feeders that secrete a tubular, chitinous structure that protects their body. They attach themselves to a hard surface and extend a crown of tentacles into the water to filter food from the current.
- Lamp shells (Brachiopoda) - There are about 350 species of lamp shells alive today. Members of this group are marine animals that resemble clams, although they are anatomically quite different from, and not closely related to, molluscs. Lamp shells live in the cold waters of the polar regions and the deep sea.
- Loriciferans (Loricifera) - There are about 10 species of loriciferans alive today. Members of this group are tiny (in many cases, microscopic) animals that live in marine sediments. Loriciferans have a protective external shell.
- Mud dragons (Kinorhyncha) - There are about 150 species of mud dragons alive today. Members of this group are segmented, limbless animals that lack external cilia. Mud dragons are marine invertebrates that inhabit the seafloor sediments.
- Mud worms (Gnathostomulida) - There are about 80 species of mud worms alive today. Members of this group are small marine animals that live in shallow coastal waters where they burrow in the sand and mud. Mud worms can survive in low-oxygen environments.
- Orthonectids (Orthonectida) There are about 20 species of orthonectids alive today. Members of this group are parasitic marine invertebrates. Orthonectides are simple, microscopic, multi-cellular animals.
- Placozoa (Placozoa) - There is one species of placazoa alive today, Trichoplax adhaerens. Placazoa are the simplest form of non-parasitic multi-cellular animals alive today. Placazoa are tiny marine animals that have a flat body that consists of an epithelium that encloses a layer of stellate cells.
- Priapulans (Priapula) - There are 18 species of priapulids alive today. Members of this group are marine worms that live in the in the mud sediments in shallow waters up to 300 feet deep.
- Ribbon worms (Nemertea) - There are about 1150 species of ribbon worms alive today. Most members of this group are marine invertebrates that live in seafloor sediments or attach themselves to hard surfaces such as rocks and shells. Ribbon worms are carnivores that feed on invertebrates such as annelids, molluscs, and crustaceans.
- Rotifers (Rotifera) - There are about 2000 species of rotifers alive today. Most members of this group live in freshwater environments although a few marine species are known. Rotifers are tiny invertebrates, less than one-half of a millimeter in length.
- Roundworms (Nematoda) - There are more than 22,000 species of roundworms alive today. Members of this group live in marine, freshwater, and terrestrial habitats and are found from the tropics to the polar regions. Many roundworms are parasitic.
- Sipunculan worms (Sipuncula) - There are about 150 species of sipunculan worms alive today. Members of this group are marine worms that live in shallow, intertidal waters. Sipunculan worms live in burrows, rock crevices, and shells.
- Velvet worms (Onychophora) - There are about 110 species of velvet worms alive today. Members of this group have a long, segmented body and numerous pairs lobopodia (short, stubby, leg-like structures). Velvet worms bear live young.
- Waterbears (Tardigrada) - There are about 800 species of waterbears alive today. Members of this group are small aquatic animals that have a head, three body segments, and a tail segment. Waterbears, like velvet worms, have four pairs of lobopodia.
Hickman C, Roberts L, Keen S. Animal Diversity. 6th ed. New York: McGraw Hill; 2012. 479 p.
Hickman C, Roberts L, Keen S, Larson A, l'Anson H, Eisenhour D. Integrated Principles of Zoology 14th ed. Boston MA: McGraw-Hill; 2006. 910 p.
Ruppert E, Fox R, Barnes R. Invertebrates Zoology: A Functional Evolutionary Approach. 7th ed. Belmont CA: Brooks/Cole; 2004. 963 p.