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, mountain tops to ocean depths.
All animals have cells that lack rigid cell walls (like those found in plant cells). Most animals reproduce sexually and are diploid (adults have cells that include two copies of their genetic material). There are between 3 and 30 million species of animals, though this remains a rough estimate. The largest subgroup of animals by far is the insects.
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.
Classification
- Kingdom: Animalia (Animals)
The Kingdom Animalia contains the following subgroups:- Phylum: Annelida (segmented worms)
- Phylum: Arthropoda (crustaceans, insects, spiders, and relatives)
- Phylum: Brachiopoda (lamp shells)
- Phylum: Bryozoa (bryozoans, ectoprocts, and moss animals)
- Phylum: Chaetognatha (arrow worms)
- Phylum: Chordata (chordates)
- Phylum: Cnidaria (cnidarians)
- Phylum: Ctenophora (comb jellies)
- Phylum: Cycliophora
- Phylum: Echinodermata (echinoderms)
- Phylum: Entoprocta (entoprocts)
- Phylum: Gastrotricha (gastrotrichs)
- Phylum: Gnathostomulida (gnathostomulids)
- Phylum: Hemichordata (hemichordates)
- Phylum: Kinorhyncha (kinorhynchs)
- Phylum: Loricifera (loriciferans)
- Phylum: Mollusca (mollusks)
- Phylum: Myxozoa (ciliated protozoans)
- Phylum: Nematoda (nematodes)
- Phylum: Nematomorpha (gordian worms and horsehair worms)
- Phylum: Nemertea (proboscis worms)
- Phylum: Onychophora (velvet worms)
- Phylum: Orthonectida (orthonectids)
- Phylum: Phoronida (phoronids)
- Phylum: Placozoa
- Phylum: Platyhelminthes (flatworms)
- Phylum: Porifera (sponges)
- Phylum: Priapula (priapulans)
- Phylum: Rhombozoa (rhombozoans)
- Phylum: Rotifera (rotifers)
- Phylum: Sipuncula (sipunculan worms)
- Phylum: Tardigrada (water bears)
References:
- Burnie D, Wilson DE. 2001. Animal. London: Dorling Kindersley. 624 p.
