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Animal

Animals
#redirect
Domain: Eukaryota #redirect : Animalia
Phyla
Subkingdom Parazoa     Porifera (sponges) Subkingdom "Agnotozoa"       Placozoa       Orthonectida       Rhombozoa Subkingdom Metazoa   "Radiata"       Cnidaria       Ctenophora (comb jellies)   Bilateria    Protostomia       Acoelomorpha       Platyhelminthes (flatworms)       Nemertina (ribbon worms)       Gastrotricha       Gnathostomulida (jawed worms)       Micrognathozoa       Rotifera (rotifers)       Acanthocephala       Priapulida       Kinorhyncha       Loricifera       Entoprocta       Nematoda (roundworms)       Nematomorpha (horsehair worms)       Cycliophora       Mollusca (mollusks)       Sipuncula (peanut worms)       Annelida (segmented worms)       Tardigrada (water bears)       Onychophora (velvet worms)       Arthropoda (insects, etc)       Phoronida       Ectoprocta (moss animals)       Brachiopoda    Deuterostomia       Echinodermata       Chaetognatha (arrow worms)       Hemichordata (acorn worms)       Chordata (vertebrates, etc)

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). Humans however are actually animals. The word "animal" derives from the Latin anima, in its sense of vital breath, and comes to English via the Latin word for animal, animal. Animalia is the plural.

Table of contents

1 Development and evolution 2 Distinguishing characteristics 3 Evolution and basal forms 4 Metazoa 5 History of classification 6 Examples 7 See also 8 External links

Development and evolution

Adult animals are typically diploid, producing small motile sperm and large non-motile eggs. In all forms the fertilized zygote initially divides to form a hollow sphere called a blastula. This then undergoes rearrangement and differentiation. Blastulae are probably representative of the sort of colonies animals evolved from; similar forms occur among other flagellates, e.g. Volvox. However none of these other groups really ever progressed further, and large multicellular forms tend to develop by progressive growth instead.

Distinguishing characteristics

Evolution and basal forms

This massive adaptive radiation may have come about because of climate change or a simple genetic innovation, and is so sudden that it is usually called the Cambrian explosion.

The sponges (Porifera) were separated from the other animals early on, and are very different. Sponges are sessile and usually feed by drawing in water through pores all over the body, which is supported by a skeleton typically divided into spicules - the cells are differentiated, but not organized into distinct groups.

There are also three problematic phyla - the Rhombozoa, Orthonectida, and Placozoa - that have an unclear position with respect to other animals. When they were first discovered, the Protozoa were considered as an animal phylum or subkingdom, but as they are generally unrelated and often as similar to plants as animals, a new kingdom, the Protista, was devised to hold them.

Metazoa

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 (the archenteron). 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 trochophore larva, which are distinguished by a special pattern of cilia. These make up a group called the Trochozoa, comprising the following:

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

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, Gnathostomulida, 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 (sea stars, sea urchins, sea cucumbers, etc)
• Phylum Hemichordata
• Phylum Chordata (vertebrates and their kin)

• Phylum Archaeocyatha (possibly sponges)
• Phylum Conulariida (possibly cnidarians)
• Phylum Conodonta (possibly chordates or near relatives thereof).
• Phylum Lobopoda (probably arthropods)
• Phylum Sclerotoma (several otherwise different forms with sclerites)
• Phylum Vendozoa (some Precambrian forms, possibly not even animal)

• Phylum Vetulicolia (probably deuterostomes)
• Unknown (A few forms like Cloudina and Hyolithes)

Examples

• ant, antelope, badger, bear, bee, beetle, bird, bison, butterfly, cat, coral, chicken, dinosaur, dog, elk, fish, fly, fox, frog, goat, horse, human, jellyfish, lion, lizard, lynx, monkey, octopus, owl, ox, parrot, penguin, pig, rabbit, rat, salamander, scorpion, seahorse, shark, sheep, sloth, snake, spider, squid, starfish, turtle, whale, wolf, worm, zebra

• Animal intelligence
• Biota
• Zoology
• Animal (Muppet)

• Tree of Life
• Animalia: A huge online vote to find the world's favourite members of kingdom Animalia.

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