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 Latinanima, in its sense of vital breath, and comes to English via the Latin word for animal, animal. Animalia is the plural.
Animals are eukaryotes, and diverged from the same group of flagellateprotozoa that gave rise to the fungi and choanoflagellates. The last are especially close relatives, with collared cells appearing only among them, the sponges, and rarely in certain other animal forms. In all these groups, called opisthokonts, 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.
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.
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:
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 pseudocoelomate 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:
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.