Animal

                                                     Animals
Animals are the group of organisms           Scientific classification

that constitute the kingdom Animalia.  Domain:  Eukaryota
Typically, they are multicellular in
composition and capable of both        Kingdom  Animalia

locomotion and responding to their                     Phyla
surroundings. Unlike plants, animals   Subkingdom Parazoa
do not photosynthesize, instead        ΚΚΚ Porifera (sponges)
consuming plants or other organisms    Subkingdom "Agnotozoa"
to grow and sustain themselves. Most   ΚΚΚΚΚ Placozoa
animals have a body plan that becomes  ΚΚΚΚΚ Orthonectida
fixed as they mature and, except in    ΚΚΚΚΚ Rhombozoa
animals that metamorphose, is          Subkingdom Metazoa
established early in their             Κ "Radiata"
development from embryos. The          ΚΚΚΚΚ Cnidaria
scientific study of animals is called  ΚΚΚΚΚ Ctenophora
zoology.                               Κ Bilateria
                                       ΚΚ Protostomia
Colloquially, "animal" often is used   ΚΚΚΚΚ Platyhelminthes (flatworms)
to refer to all animals other than     ΚΚΚΚΚ Nemertina (ribbon worms)
humans and rarely to refer to animals  ΚΚΚΚΚ Gnathostomulida (jawed worms)
not classified as metazoan (see        ΚΚΚΚΚ Gastrotricha
"Metazoa" below). The word "animal"    ΚΚΚΚΚ Rotifera (rotifers)
derives from the Latin anima, in its   ΚΚΚΚΚ Priapulida
sense of vital breath, and comes to    ΚΚΚΚΚ Kinorhyncha
English via the Latin word for         ΚΚΚΚΚ Loricifera
animal, animalis. Animalia is the      ΚΚΚΚΚ Acanthocephala
plural.                                ΚΚΚΚΚ Entoprocta
                                       ΚΚΚΚΚ Nematoda (roundworms)
                                       ΚΚΚΚΚ Nematomorpha (horsehair worms)
                                       ΚΚΚΚΚ Cycliophora
                                       ΚΚΚΚΚ Mollusca (mollusks)
                                       ΚΚΚΚΚ Sipuncula (peanut worms)
                                       ΚΚΚΚΚ Annelida (segmented worms)
                                       ΚΚΚΚΚ Tardigrada (water bears)
                                       ΚΚΚΚΚ Onychophora (velvet worms)
                                       ΚΚΚΚΚ Arthropoda (insects, etc)
Development and evolution              ΚΚΚΚΚ Phoronida
                                       ΚΚΚΚΚ Ectoprocta (moss animals)
Animals are eukaryotes, and diverged   ΚΚΚΚΚ Brachiopoda
from the same group of flagellate      ΚΚ Deuterostomia
protozoa that gave rise to the fungi   ΚΚΚΚΚ Echinodermata
and choanoflagellates. The last are    ΚΚΚΚΚ Chaetognatha (arrow worms)
especially close relatives, with       ΚΚΚΚΚ Hemichordata (acorn worms)
collared cells appearing only among    ΚΚΚΚΚ Chordata (vertebrates, etc)
them, the sponges, and rarely in
certain other animal forms. In all these groups motile cells (cells that
propel themselves) have a single posterior flagellum with similar
ultrastructure.

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

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.

Evolution and basal forms

Except for a few exceptionally questionable trace fossils, the first forms
that might represent animals appear in the fossil record around the end of
the Precambrian. These are called Vendian Biota and are exceedingly
difficult to relate to later forms. Other than them, virtually every phylum
makes a more or less simultaneous appearance during the Cambrian.

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

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 (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 trocophore
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)

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),
Nematomorpha (horsehair worms), 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, 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 (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 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)

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.