Animal

Animals are the group of organisms that constitute the kingdom Animalia. Typically, they are multicellular in composition and capable of both locomotion and responding to their surroundings. Unlike plants, animals do not photosynthesize, instead consuming plants or other organisms to grow and sustain themselves. Most animals have a body plan that becomes fixed as they mature and, except in animals that metamorphose, is established early in their development from embryos. The scientific study of animals is called zoology.

Colloquially, "animal" often is used to refer to all animals other than humans and rarely to refer to animals not classified as metazoan (see "Metazoa" below). The word "animal" derives from the Latin anima, in its sense of vital breath, and comes to English via the Latin word for animal, animalis. Animalia is the plural.

Table of contents

2 Distinguishing characteristics 4 Metazoa 5 History of classification 6 Examples

Distinguishing characteristics

The most notable distinction of animals lies in the way the cells are held together. Instead of being simply stuck together or held in place by thick walls, animal cells are linked by septate junctions, composed mainly of elastic proteins - collagen is characteristic - that make up the extracellular matrix. Sometimes this is calcified to form shells, bones, or spicules, but otherwise it's fairly flexible and can serve as a framework, upon which cells can move about and be reorganized.

Metazoa

Aside from these, all animals belong to a monophyletic group called the Metazoa (called the Eumetazoa when the name Metazoa is used for all animals), characterized by a digestive chamber and separate cell layers that differentiate into various tissues. Distinguishing features of the Metazoa include a nervous system and muscles.

The simplest Metazoa are radially symmetric and diploblastic, that is, they have two germ layers. The outer layer (ectoderm) corresponds to the surface of the blastula and the inner layer (endoderm) is formed by cells that migrate into the interior. It then invaginates to form a digestive cavity with a single opening. This form is called a gastrula or planula when it is free-swimming. The Cnidaria (jellyfish, anenomes, corals, etc) are the main diploblastic phylum; the Ctenophora (comb jellies) may also belong here. The Myxozoa, a group of microscopic parasites, have been considered reduced cnidarians but may instead be derived from the Bilateria.

The remaining forms comprise a group called the Bilateria, since they are bilaterally symmetric (at least to some degree), and are triploblastic. The blastula invaginates without filling in first, so the endoderm is simply its inner lining, and the interior then fills in to become a third layer (mesoderm) between the others. Like tissues are grouped into organs. The simplest of such animals are the Platyhelminthes (flatworms), which may be paraphyletic to the higher phyla.

The vast majority of the triploblastic phyla form a group called the Protostomia. These phyla all have a complete digestive tract (including a mouth and an anus), with the mouth developing from the archenteron and the anus arising later. The mesoderm arises as in the flatworms, from a single cell, and then divides to form a mass on each side of the body. Usually there is a hollow space around the gut, called the coelom, arising from a split within the mesoderm, or at least some reduced version thereof (eg a pseudocoelom, where the split occurs between the mesoderm and endoderm, common in microscopic forms).

Some of the main protostome phyla are united by the presence of trocophore larva, which are distinguished by a special pattern of cilia. These make up a group called the Trochozoa, comprising the following:

• Phylum Mollusca (snails, clams, squids, etc)
• Phylum Sipuncula
• Phylum Annelida (segmented worms)

Traditionally the Arthropoda - the largest animal phylum including insects, spiders, crabs, and kin - and two small phyla closely related to it, the Onychophora and Tardigrada, have been considered close relatives of the Annelida on account of their common segmented body plan (the Articulata hypothesis). This relationship is now in doubt, and it appears that instead they belong with various pseudocoeolomate worms - the Nematoda (roundworms), Kinorhyncha, Loricifera, and Priapulida - which share with them ecdysis and several other characteristics. This group is called the Ecdysozoa.

There are various pseudocoelomate protostomes that are hard to classify because of their small size and reduced structure. The Rotifera and Acanthocephala are closely related to each other and probably belong near the Trochozoa. Other groups include the Gastrotricha, Entoprocta, and Cycliophora. The last was discovered only recently, and as little investigation has been done into the marine world more will probably turn up. Most of these were originally grouped as the phylum Aschelminthes, together with the Nematoda and others, but they do not appear particularly closely related to each other.

The Brachiopoda (lamp shells), Ectoprocta (=Bryozoa, literally moss animals), and Phoronida form a group called the Lophophorata, thanks to the shared presence of a fan of cilia around the mouth called the lophophore. The evolutionary relationships of these forms are very unclear - the group has even been considered among the deuterostomes, and may be paraphyletic. They are most likely related to the Trochozoa, however, and the two are often grouped as the Lophotrochozoa.

The Deuterostomes differ from the Protostomes in various ways. They also have a complete digestive tract, but in this case the archenteron develops into the anus. The mesoderm and coelom do not form in the same way, but rather through evagination of the endoderm called enterocoelic pouching. And, finally, the embryonic cleavage is different. All this suggests that the two lines are separate and monophyletic. The Deuterostomes include:

• Phylum Chaetognatha
• Phylum Echinodermata (starfish, sea urchins, sea cucumbers, etc)
• Phylum Hemichordata
• Phylum Chordata (vertebrates and their kin)

There are also some extinct animal phyla that, without much knowledge of their embryology or internal structure, are very difficult to place. These are mostly from the cambrian period, and include

• Phylum Archaeocyatha (possibly sponges)
• Phylum Vendozoa (some Precambrian forms, possibly not even animal)
• Phylum Vetulicolia (probably deuterostomes)
• Unknown (A few forms like Cloudina and Hyolithes)

History of classification

In Linnaeus' original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.

Examples

Some well-known types of animals, listed by their common names:

• Ant, Antelope, badger, Bear, Bee, Beetle, Bird, Butterfly, Cat, Coral, Cow, Chicken, Dinosaur, Dog, Fish, Fly, Frog, Goat, Horse, Human, Jellyfish, Lion, Lizard, Lynx, Monkey, Octopus, Owl, Ox, Parrot, Penguin, Pig, Rabbit, Rat, Salamander, Scorpion, Seahorse, Shark, Sheep, Snake, Spider, Squid, Starfish, Turtle, Whale, Wolf, Worm

See also animal intelligence, zoology

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