An animal is an organism of the kingdom Animalia, usually characterized by heterotrophy, motility, and cells with well defined nuclei and with cell membranes but no cell walls. Sometimes humans (and perhaps other ellogous races) use the word specifically to refer to beasts—that is, to nonhuman, alogous animals—but technically humans are animals as well, as surely as crickets and crocodiles. At the other extreme, many laymen use the word "animal" to refer to any fauna, even that pertaining to other empires. This, too, is technically inaccurate; only organisms of the kingdom Animalia are technically animals.
Animals are classified within the empire of Telluria, kingdom Animalia (sometimes, though now infrequently, called Metazoa). Sometimes Animalia is lumped with certain related protists in the superkingdom Filozoa, which in turn is combined with the single-celled ichthyosporeans in the magnakingdom Holozoa. Going further up the iffier branches of the taxonomic tree, Holozoa is sometimes included along with fungi in the infradomain of Opisthokonta, which together with amœbæ and their relatives comprises the subdomain of Unikonta.
The kingdom Animalia is itself divided into two subkingdoms: Parazoa and Eumetazoa, the latter being characterized by differentiated tissues and some degree of symmetry, both of which parazoans, the more "primitive" of the two subkingdoms (in the sense that they are probably closer to the animals' common ancestor), lack. While Parazoa contains only two phyla, Porifera (the sponges) and Placozoa (flat amorphous animals found in coastalareas), Eumetazoa contains the majority of the several dozen extant animal phyla. Some evidence suggests, however, that placozoans may be more closely related to eumetazoans than to parazoans, to reflect which Eumetazoa is relegated to an infrakingdom that pertains, along with Placozoa, to the subkingdom Epitheliozoa.
Among Euterran animals, the largest pertain to the phylum Chordata, and more specifically to the subphylum Vertebrata; few other animals rival the larger vertebrates in size. The only animal of another phylum to give the vertebrates a run for its money is the giant squid, but even it, though larger than most vertebrates, is surpassed by some of the largest whales. The largest Euterran phylum in terms of number of species, rather than in size of individuals, is the phylum Arthropoda; this phylum includes the myriad species of insects, but also arachnids and other classes, and includes more than eighty percent of known animal species. Other widespread and well-known animal phyla include Mollusca, which encompasses such diverse organisms as snails, clams, and octopuses; Echinodermata, to which pertain sea urchins and sea cucumbers; and Cnidaria, among the members of which are the sea jellies and corals.
Animal cells have a number of unique features setting them apart from those of other Tellurian organisms. Unlike most eukaryotic cells, they completely lack cell walls, being the only Euterran metabionts to do without this feature. While the absence of cell walls means that animals have to resort to more complicated means to hold their structures, it also allows for more malleability and is a contributing factor to the animals' greater mobility relative to plants and fungi. Animal cells also tend to have smaller vacuoles than plant or fungal cells, though these features are not entirely absent.
While they do not have the cell walls common to all other multicellular Tellurians, however, animal cells do have some unique features of their own. One significant feature confined to animal cells is the centrosome, a bicylindrical organelle that plays a role in cell division, organizing the microtubules that sort other cellular components into the daughter cells.
On a larger scale, animal body plan varies widely by phylum. Most animals have some sort of orifices through which they take in nutrients, be they pores like in the sponges or mouths like in eumetazoans; the placozoans, however, are something of an exception, forming temporary digestive cavities on their ventral surfaces but having no permanent orifices there. Most animals apparently form from two embryonic cell layers, the endoderm and the ectoderm, the products of which are still distinguishable in the mature organisms, but here the sponges are an exception. (Depending on the exact cladistics involved, the placozoans may also be an exception, depending on whether they're more closely related to sponges or to eumetazoans; if the former, the two cell layers of the placozoans may be similar to those of eumetazoans but not truly homologous.) Eumetazoans have groups of cells differentiated into tissues such as muscle and nerve, but sponges and placozoans do not.
Animals almost certainly evolved from a marine flagellate about six hundred million years ago. Beyond that, however, details remain hazy. Animals probably evolved multicellularity independently of other metabionts, producing a multicellular common ancestor sometimes called the Urmetazoan, but precisely when and how this happened remains unknown; a fossil that seemed to show the embryo of an early animal was recently reëvaluated as a spore body of an animal ancestor instead. In any case, the oldest remaining indubitable fossils of animals on True Earth resemble modern cnidarians, though some older trace fossils suggested the existence of some vermiform organism. The enigmatic Ediacaran biota may have included animals representing a wider variety of phyla, though their exact relations remain unclear, and it's still disputed which if any of these organisms represent animals at all. In any case, the vaguely defined small shelly fauna that succeeded the Ediacaran biota were definitely animals.
The diversity of animals, or at least of those that left surviving fossils, flourished during the Cambrian Period, leading to a phenomenon sometimes called the Cambrian explosion. When the weird fossils from this period were first discovered, they seemed to include a variety of animals unlike any that existed today, representing a plethora of short-lived phyla. Later investigations showed that some of the seemingly sui generis Cambrian animals may have been members or precursors of modern phyla after all; the bizarre Opabinia and Anomalocaris, for instance, may have been related to arthropods, and Hallucigenia, which, befitting its name, seemed one of the weirdest animals of all, was later discovered to have been reconstructed upside-down, and in its proper posture wasn't so weird after all and may have been a primitive velvet worm, though this is still debated. Between this and further discoveries of Precambrian animals, it became less certain just how much of a drastic explosion the Cambrian explosion really was. Be that as it may, by the end of the Cambrian all major modern animal phyla were apparently represented, not excluding the chordates.
It was another hundred million years or so before animals left the oceans and began to colonize the land, and when they did it was the arthropods that were first. The oldest evidence of land animals consists of trace fossils called Protichnites, but may implicate relatives of the myriapods (centipedes, millipedes, and their kin). It was arthropods, too, that were the first animals to take to the air, with flying insects probably appearing more than three hundred and fifty years ago during the late Devonian or early Carboniferous. The vertebrates, which had developed in the ocean in the form of various clades of fish, made it to land rather later than the arthropods, when some labyrinthodonts (early amphibians) about 360 million years ago became semiaquatic, and their descendents later became fully terrestrial. Once the vertebrates did reach land, however, they became the largest animals there, culminating in the dinosaurs during the Mesozoic Era, which included the largest land animals ever to live (although not the largest animals of all; that record is believed to go to the still-extant ocean-dwelling blue whale). The large dinosaurs died out during the K-T extinction event, to be succeeded as the largest land animals by the mammals; today the largest land animal is the mammalian elephant. (Some smaller dinosaurs did survive the mass extinction, and their descendants still flourish today as birds.)
Euterran animals, at least, are exclusively heterotrophic, typically preying on plants or other animals or on their remains (though a few animals may feed on fungi or microorganisms). Animals that prey on other (motile) animals (carnivores) must rely on either speed or on stealth and surprise to catch their mobile and reactive prey. Animals that prey on immotile plants (herbivores) need no such measures to obtain their food, though they may still find speed or stealth useful to avoid being preyed on by carnivores. The same is true of scavenging animals that eat the remains of animals that were killed by other predators or died of other causes.
A few animals take advantage of mutualistic relationships with bacteria and other microörganisms to be able to process food they wouldn't be able to digest on their own. Cellulose, for instance, one of the principal structural components of plant cells, is indigestible by most animals, but ruminants such as cows and goats can digest it with the aid of microbes in their guts; the same is true of termites (though the particular microörganisms involved are different). Even in humans, "gut flora" help break down certain carbohydrates, though humans can survive without them.
Almost all animals undergo sexual procreation. Some animal species have distinct male and female sexes, one of each is necessary for reproduction, while others are hermaphroditic. Organisms of some animal species can even change sex under certain circumstances. Through a process called meiosis, the animals form special reproductive cells, or gametes, flagellated cells called spermatozoa (produced by the male in duosexual species) and larger cells called ova (produced by the female). A spermatozoön "fertilizes" an egg, combining with it to form a zygote, a single cell with the capacity of dividing and multiplying to form an animal like its parents. (The intermediate stage, when the animal-to-be comprises more than one cell but is still in the process of development, is called an embryo.) Eumetazoa generally have dedicated reproductive tissues or organs, but even the simple sponges reproduce sexually by releasing sperm into the water to be carried to other nearby individuals.
Many animals can also reproduce through various asexual means. While placozoans are capable of sexual reproduction, they more often multiply by budding. Many cnidarians are also capable of budding, and a number of different animals can also reproduce through fragmentation. Even some vertebrates (as well as some arthropods and nematodes) are capable of asexual reproduction through parthenogenesis.
- Animal on Wikipedia