Animals (Metazoa) are a group of living organisms that include 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 somewhere 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'll focus mainly 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.
Before discussing what animals are, it helps to understand what animals are not. Animals are not plants. Plants includes organisms such as green algae, mosses, ferns, confiers, cycads, ginkos, and flowering plants. Animals also are not fungi (yeasts, moulds and mushrooms). Other non-animal groups of organims include protists (red algae, ciliates, and various unicellular microorganisms), bacteria (tiny prokaryotic microorganisms), and archaea (single-celled microorganisms).
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 include arthropods, chordates, cnidarians, echinoderms, molluscs, sponges and numerous other lesser-known creatures. Animals that are most familiar to us are, in turn, members of these high level groups. For example, insects, crustaceans, arachnids and horseshoe crabs are arthropods. Amphibians, birds, reptiles, mammals, and fishes are vertebrates. Jellyfish, corals and anemones are cnidarians.
This vast diversity makes it difficult to draw generalizations, but there are a few commonalities that describe most (if not all) animals. For example, most animals are multicellular organisms that are capable of locomotion. Additionally, animals are heterotrophs which means they rely on consuming other organisms to obtain their nourishment. With the exception of the sponges, animals have bodies that are differentiated into tissues that serve some function. These tissues are, in turn, organized into organ systems.
Animals lack rigid cell walls that are characteristic of plants. Animals have eukaryotic cells (their cells contain a nucleus and other structures called organelles that are enclosed within membranes). Most animals reproduce sexually by means of differentiated eggs and sperm. Most animals are diploid (the cells of adults contain two copies of their genetic material). Animals go through different stages of development (these 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:
- 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, demo sponges and glass sponges. Sponges are primitive multi-celluar animals that have no digestive system, circulatory system or 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 attached 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 tiny, 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 mouthparts of Norwegian lobsters in the North Atlantic. Cycliophorans have a body that is divided into a mouthlike structure called a buccal funnel, an oval midsection and a stalk that ends in an adhesive disc that is used to attach the animal to the setae on the loster's mouthparts.
- 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 nor do they have a circulatory or respiratory system. Oxygen and nutrients must pass through their body wall by means of diffusion. This limits their body structure to being flat.
- Gastrotrichs (Gastrotricha) - There are about 500 species of gastrotrichs alive today. Most members of this group are freshwater species, although there are some marine and terrestrial species as well. Gastrotrichs are microscopic animals with transparent bodies and cilia on their bellies.
- 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 hemicordates alive today. Members of this group include acorn worms and pterobranchs. Hemichordates are worm-shaped animals, some of which live in tubular structures (also known as a coenecium).
- Horshoe 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 bear some resemblence to clams, though they are anatomically quite different and not closely related to any molluscs. Lamp shells live in cold waters of the polar regions and the deep sea.
- Loriciferans (Loricifera) - There are about 10 secies of loriciferans alive today. Members fo 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 sgemented, limbless aniamls 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 the 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 very 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-celluar 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 proboscis 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. Rotivers are tiny invertebrates, less than half 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, holes in rocks, 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.
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