Sea nettles, Chrysaora quinquecirrha
Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. Animals are generally considered to be multicellular organisms that are capable of locomotion in response to their environment (motile), are required to ingest or eat and swallow other organisms to gain proper nutrition (heterotropic), contain within each cell genetic material organized as two sets of chromosomes within a membrane-bound nucleus (eukaryotic), develop through a blastula (hollow ball) stage, and integrate muscle tissue, nervous tissue, and collagen into their body. Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on. Animals include mammals, birds, reptiles, amphibians, fish, spiders, crabs, star fish, sea urchins, snails, sponges, earthworms, and many, many more.
Although scientifically humans are animals, in everyday usage, animal often refers to any member of the animal kingdom that is not a human being, and sometimes excludes insects (although including such arthropods as crabs). The common distinction made between animals and humans likely reflects the special status people accord themselves as the pinnacle of the natural world, and indeed stewards of creation, and the fact that humans also are defined in religious, spiritual, moral, social, and psychological terms. Indeed, many religions consider humans to uniquely have a soul or spirit that remains after death of the physical body.
Animals show an amazing diversity of size. The bee hummingbird, the smallest living bird, weighs less than 2 grams, and the goby fish of the Philippines and the Cuban tree frog are less than 1/2 of a centimeter. On the other hand, the great blue whale grows to 100 feet in length, and may weigh more than 130 tons. Throughout the great range of sizes, many animal species reproduce through a sexual interaction between male and female partners in which one set of chromosomes from each parent fuse to form a zygote that develops into a new individual.
The name animal comes from the Latin word animal, of which animalia is the plural, and ultimately from anima, meaning vital breath or soul.
For a long time, living organisms were divided only into the animal kingdom (Animalia) and the plant kingdom (Plantae). These were distinguished based on such characteristics as whether the organisms moved, had body parts, and took nourishment from the outside (animals), or were stationary and able to produce their own food by photosynthesis (plants). However, many organisms remained difficult to classify as plant or animal, and seemed to fit both, or neither, kingdom. Subsequently, more kingdoms were recognized, such as the five kingdom system of Protista, Monera, Fungi, Plantae, and Animalia, or a system that places three domains above the kingdoms: Archaea, Eubacteria, and Eukaryota. (See taxonomy.)
Kingdom Animalia has several characteristics that set it apart from other living things placed in other kingdoms. Animals are eukaryotic (genetic material is organized in membrane-bound nuclei) and multicellular (comprised of more than one cell), which separates them from bacteria and most protists. They are heterotrophic (unable to synthesize their own food by photosynthesis or chemosynthesis, and feed by consuming other organisms), which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. Furthermore, while fungi are also multicellular, heterotropic eukaryotes, they are absorptive heterotrophs, secreting enzymes onto their food, whereas animals generally take in the food though ingestion or eating and swallowing.
The animal kingdom is further separated into phyla, which are major groupings of animals according to some basic body plan. For example, all sponges are classified in the phylum Porifera. Taxonomists recognize about 37 to 38 phyla of living animals, with several additional phyla having once existed. All major bilaterian phyla with fossilizable hard parts made their first appearance in the fossil record during the Cambrian period about 500 million years ago (Gould 2002). Animal phyla are further divided into classes, orders, families, genera, and species.
In addition, some superphyla are recognized, grouping various phyla with common characteristics, as well as various superkingdoms, subkingdoms, sub-classes, and so forth.
The classification schemes for animals reflect the diversity of the organisms themselves. One possible biological classification of animals is presented in the box at the top of this article. Another classification is offered by Margulis and Schwartz (1998):
- Subkingdom Parazoa
- Phylum Placozoa (only one species, the marine Trichoplax adhaerens)
- Phylum Porifera (sponges)
- Subkingdom Eumetazoa (cells are organized into organ or organ systems)
- Phylum Cnidaria/Coelenterata (hydra, jellyfish, sea anemones, corals)
- Phylum Ctenophora (comb jellies)
- Phylum Platyhelminthes (flatworms)
- Phylum Gnathostomulida (jaw worms)
- Phylum Rhombozoa (dicyemids and heterocyemids)
- Phylum Orthonectida (small parasites of marine mollusks and polychaete worms)
- Phylum Nemertina (ribbon worms)
- Phylum Nematoda (roundworms, thread worms)
- Phylum Nematomorpha (horsehair worms, Gordian worms)
- Phylum Acanthocephala (thorny-headed worms)
- Phylum Rotifera (rotifers)
- Phylum Kinorhyncha (small marine animals)
- Phylum Priapulida (benthic, marine worm-like animals)
- Phylum Gastrotricha (unsegmented, wormlike animals of freshwater or marine benthos)
- Phylum Loricifera (minute marine animals found attached to rocks and shells)
- Phylum Entoprocta (small filter feeders found along ocean coasts)
- Phylum Chelicerata (spiders, scorpions, mites, ticks, horseshoe crabs)
- Phylum Mandibulata (insects, millipedes, centipedes)
- Phylum Crustacea (crayfish, shrimp, copepods, isopods, and relatives)
- Phylum Annelida (segmented worms, including earthworms, and polychaetes)
- Phylum Sipuncula (peanut worms)
- Phylum Echiura (spoon worms)
- Phylum Pogonophora (bearded tube worms)
- Phylum Mollusca (mussels, clams, snails, slugs, squids)
- Phylum Tardigrada (water bears)
- Phylum Onycophora (velvet worms)
- Phylum Bryozoa (moss animals)
- Phylum Brachiopoda (lampshells)
- Phylum Phoronida (horseshoe worms)
- Phylum Chaetognatha (arrow worms)
- Phylum Hemichordata (acorn worms)
- Phylum Echinodermata (starfish, sea cucumbers, sea urchins)
- Phylum Urochordata (tunicates, sea squirts)
- Phylum Cephalochordata (amphioxus, lancelets)
- Phylum Craniata (fishes, amphibians, reptiles, birds, mammals)
Superphyla are also recognized in this scheme, such as the superphylum Arthropoda that includes the phyla Chelicerata, Crustacea, and Mandibulata. Subphyla exist, too, such as the subphylum Vertebrata, which includes such classes as Chondrichthyes (shark, skate, ray), Osteichthyes (bony fish), Amphibia (frogs, salamanders), Reptilia (turtle, snake, lizard, alligator), Aves (birds), and Mammalia (marsupials, placentals, platypus, dog, human). Some other schemes consider arthropods as a phylum (Arthropoda), rather than a superphylum, and Chelicerata and Mandibulata are considered subphyla of Arthropoda, rather than phyla themselves. Some taxonomists see two phyla, Endoprocta and Bryozoa, under a subkinggom Pseudocoelomata, whereas others put both into the one phylum Bryozoa.
Yet other schemes may organize phyla according to whether they are radially or bilaterally symmetrical, or by characteristics of the formation of the body cavity and the origin of the anus and mouth. For example, the superphylum Deuterostomes are those whose anus develops from the first cavity formed during embryo development and include such phyla as Chaetognatha, Hemichordata, and Echinodermata. Different schemes may also use different names for phyla, such as Craniata versus Chordata. This gives some idea of the diversity of organizational schemes for animals.
There are about 40,000 known species of vertebrates and more than 1 million known species of invertebrates, but it is generally established that only a small percentage of all animal species are known. Approximately 1.8 million species of animals and plants have been identified (excluding the diverse kingdoms of fungi, bacteria, and other unicellular organisms), but some biologists estimate there may be more than 150 million species of living things on the earth. More than one-half of those identified are insects (about 57 percent), and nearly half of all insect species are beetles. There are about 100,000 known species of mollusks, and 9,000 species of coelenterates. There are also approximately 9,000 named species of birds, 27,000 known species of fish, and a ledger of about 4,000 or so mammalian species. These vertebrate groups have been diligently catalogued, unlike insects which rank among the most uncounted groups of organisms.
History of classification
Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus in the first hierarchical 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 or the Craniata, whereas the various other forms have been separated out. Some organisms, such as the microscopic protozoa, were originally considered animals because they move, but are now treated separately. Since Darwin, biologists have begun emphasizing presumed evolutionary relationships. See taxonomy for a greater elaboration of the history of classification.
The structure of animals may exhibit radial symmetry or bilateral symmetry. Radial symmetry occurs when the body parts radiate from the center, such as seen in the phylum Cnidaria (Coelenterata), with hydra, jellyfish, and coral. Radial symmetry is generally found in organisms that are sessile and are dependent on the water currents to move about or bring food to them. Bilateral symmetry involves the right and left sides of the organism being roughly mirror images of each other, with a head end and a posterior. Most animals have bilateral symmetry, as it is adapted for active movement.
With a few exceptions, most notably the sponges (phylum Porifera), animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and a nervous system, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans, where the former is used for animals in general.
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development, it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.
Reproduction and development
Nearly all animals undergo some form of sexual reproduction. Adults are diploid, although occasionally polyploid forms exist. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.
A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers—an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.
Animals grow by indirectly using the energy of sunlight. Plants use this energy to turn air into simple sugars using a process known as photosynthesis. These sugars are then used as the building blocks that allow the plant to grow. When animals eat these plants (or eat other animals that have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.
Origin and fossil record
The first fossils that might represent animals appear towards the end of the Precambrian, around 600 million years ago, and are known as the Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most animals with known phyla make a more or less simultaneous appearance during the Cambrian Period, about 570 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.
Groups of animals
The sponges (Porifera or "pore-bearer") are represented in the fossil record as far back as the early Precambrian, more than 600 million years ago. They are all aquatic—freshwater and marine—and show a diversity of form. However, sponges lack the complex organization found in most other phyla. Their cells are differentiated, but not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores. They appear to lack symmetry.
Among the Eumetazoan phyla, two are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. These are the coelenterates (phylum Cnidaria), which include sea anemones, corals, sea anemones, and jellyfish; and the Ctenophora, or comb jellies. Organisms of both phyla have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny phylum Placozoa is similar, but individuals do not have a permanent digestive chamber.
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, in other words, all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however. For instance, adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.
Some taxonomists, relying on genetic studies, differentiate most of the Bilateria into four main groups, representing four lineages:
In addition to these, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered protozoa, are now believed by some to be Bilateria that have developed prior to the four groups as well.
Deuterostomes differ from those Bilateria termed protostomes, in several ways. In both cases, there is a complete digestive tract. However, in protostomes the first cavity formed during embryo development (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes, this is reversed, with the anus developing from the initial opening formed during embryo development. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as sea stars, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.
In addition to these, the deuterostomes also include the phylum Hemichordata or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group. The phylum Chaetognatha, or arrow worms, may also be deuterostomes, but this is less certain. The phyla Urochordata (tunicates, sea squirts) and Cephalochordata (amphioxus, lancelets) are also placed within the deuterostomes.
The Ecdysozoa are protostomes and are named after the common trait of growth by molting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the Nematoda or roundworms, the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, which are visible to the unaided eye, and the Kinorhyncha, Priapulida, and Loricifera, which are all microscopic. These groups have a reduced coelom, called a pseudocoelom. Some taxonomists lump the phyla Nematoda, Nematomorpha, Acanthocephala, Rotifera, and Kinorhyncha as "Pseudocoelomates" rather than Ecdysozoas.
The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but now there is speculation that they developed from more complex ancestors.
The other Platyzoan phyla are microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora. These groups share the presence of complex jaws, from which they are called the Gnathifera.
Some taxonomic schemes lump the Platyhelminthes, Orthonectida, and Nemertina as "Acoelomates."
The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida. The former includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods, because they are both segmented. Now this is generally attributed to convergent evolution, owing to many morphological and genetic differences between the two phyla.
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore. These later phyla were traditionally grouped together as the lophophorates, but now they are generally considered paraphyletic, some closer to the Nemertea and some to the Mollusca and Annelida. They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the [[Phoronida]*], and possibly the Bryozoa, or moss animals.
Listed below are some well-known types of animals, listed by their common names:
- Gould, S. J. 2002. The Structure of Evolutionary Theory. Cambridge, MA: Belknap Press of Harvard University Press.
- Klaus Nielsen. 2001. Animal Evolution: Interrelationships of the Living Phyla (2nd edition). Oxford University Press.
- Knut Schmidt-Nielsen. 1997. Animal Physiology: Adaptation and Environment (5th edition). Cambridge University Press.
- Margulis, L. & Schwartz, K.V. 1998. Five Kingdoms. An Illustrated Guide to the Phyla of Life on Earth. New York: W.H. Freeman.
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