About Animals

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A living organism; one of the main kingdoms of biological classification, Animalia, containing all vertebrates and invertebrates. The term animal is sometimes used only for four-legged creatures (mammals, reptiles, and amphibians), but correctly fish and birds are also animals, as are insects, spiders, crabs, snails, worms, starfish, sponges, corals, jellyfish, and many other groups.

All animals obtain their nourishment by eating other living organisms or the remains of living organisms, and most can respond quickly to changes in their surroundings by moving part of their body or their entire body.

Any member of the Kingdom Animalia, distinct from the other three kingdoms: Monera, Plantae and Fungi. The bodies of animals consist of one or more cells which, unlike those of the Plantae and Fungi, lack cell walls and, unlike Monera, have the chromosomal material of each cell enclosed within a nucleus.

The most obvious characteristic of animals is that, unlike plants, they are unable to use the energy of sunlight to power their life processes (autotrophy) and must obtain their nourishment by feeding on other living organisms, the excretions or secretions of these, or their dead remains.

Animals comprise three subkingdoms: the Protozoa, which consist of single cells or simple colonies of these; the Parazoa or sponges, which have a body made up of many cells but lack nerve cells, muscle cells and definite organs; and the Metazoa, which have bodies that are organized into definite tissues and organs.

The Metazoa is a very diverse group of many phyla, of which the more familiar are the Coelenterata (or Cnidaria) (jellyfishes, anemones, corals); the Nematoda (roundworms, threadworms); Platyhelminthes (flatworms, flukes, tapeworms); Mollusca (bivalves, snails, slugs, octopuses, squids); Annelida (ragworms, earthworms, leeches); Arthropoda (insects, centipedes, millipedes, crustaceans, spiders, scorpions); Echinodermata (starfishes, sea-urchins, crinoids); and Chordata (sea-squirts, fishes, frogs, reptiles, birds, mammals).

Most animals are free-living and can move rapidly towards a favourable stimulus and away from an unfavourable one. Some aquatic animals, (such as sponges, corals and barnacles), spend their adult lives fixed in one place but can respond to stimuli by extending or retracting their bodies.

Their larvae are usually free-living and are dispersed by swimming and by water currents. All metazoans (except some degenerate parasites) have a mouth, but coelenterates and platyhelminths lack an anus, possession of which enables food to be digested and absorbed progressively as it passes through the gut.

All animals require to take in oxygen and to get rid of carbon dioxide. In aquatic animals this exchange of dissolved gases takes place through the skin or through gills, which are branched, thin-walled extensions of the skin. Some terrestrial animals, such as scorpions and snails, also possess gills, protected in a damp pouch.

Others, such as insects and spiders have a system of air-filled tubes (tracheae) that branch to every part of the body. Gas exchange in terrestrial chordates is by means of lungs, which are extremely subdivided bags that communicate to the atmosphere via the nose and mouth.

The body fluids of annelids, molluscs, arthropods and chordates are circulated through the body by one or more pumping devices known as hearts. The heart reaches its greatest development in the chordates, which are also the only animals to have a completely closed circulatory system (heart to arteries to capillaries to veins to heart).

All metazoans are sensitive to a range of chemical stimuli: many have chemosensory organs in or around the mouth or on tentacles or antennae arising from the head.

Eyes occur in some annelids and molluscs and in most arthropods and chordates. Organs sensitive to the pitch of sounds are restricted to some arthropods and most chordates. All metazoans have a nervous system, usually with a concentration of nervous tissue near the anterior end and several strands extending backwards from this.

A nervous system, with brain, is found in annelids, arthropods, molluscs and chordates. All animals have, to some extent, the capacity to modify their behaviour on the basis of past experience. In most phyla, this learning ability is slight. It is best developed in the chordates, particularly mammals.

Written by Ronald Strahan

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