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A class of the phylum Radiozoa in the Actinopoda. The kingdom Protozoa contains 18 phyla. One of the parvkingdoms (a hierarchical classification between kingdom and superphylum that is controversial and not officially recognized as such) is the…

An order of ectoparasitic annelid worms consisting of one species, Acanthobdella peledina, which feeds on the dermal tissues of freshwater salmonid fishes in the far regions of Northern Europe. The order Acanthobdellida is taxonomically odd in that…

A distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class,…

A taxon of marine protozoans belonging to the Acantharea. Acanthometrida (also known as Arthracanthida) is a taxonomic grouping (sometimes considered as an order or subclass) placed within the Acantharea (Acantharia) taxon. All members are…

A taxon of arachnids (traditionally considered a subclass of the Arachnida), comprising the mites and ticks. The Acari (also Acarina) consist of two superorders of small to minute arachnids. They have two body regions (Fig. 1)—a headlike gnathosoma…

An order of actinopterygian (ray-finned) fishes represented by the sturgeons and paddlefishes. The order Acipenseriformes, the only extant order of about 10–12 orders of chondrosteans (subclass Chondrostei), probably originated from palaeonisciform…

A group of small, soft-bodied, ciliated worms with the form (“morph”) of acoel turbellarians. The Acoelomorpha is now ranked as a phylum, distinct from other turbellarian worms of the phylum Platyhelminthes. Its relation to other animals, however,…

A subtaxon of burrowing barnacles in the subclass Thecostraca. All members of the Acrothoracica burrow into shells of mollusks and thoracican barnacles, echinoderm tests, bryozoans, dead coral, and limestone. The entrance to the burrow is slitlike.…

An order of inarticulated brachiopods. Together with Lingulida and Siphonotretida, Acrotretida is one of the three orders of inarticulated brachiopods currently classified in class Lingulata in subphylum Linguliformea (phylum Brachiopoda).…

An order of the class Anthozoa, commonly known as the sea anemones. Actiniaria contains approximately 1100 species. They are strictly marine, occurring from northern to southern polar latitudes and inhabiting regions that range from the intertidal…

An order of Heliozoia. These protozoans are characterized by stiff arms radiating from the central cell mass. Each arm is supported internally by an array of microtubules which terminate on the surface of nuclei. The organisms are found in…

A class of Sarcodina; or in some modern schemes a superclass (Actinopoda) within the phylum Sarcomastigophora. Some of these protozoans have more or less permanent pseudopodia, composed of a central shaft of microtubules surrounded by a thin…

A class of teleostom fishes commonly known as the ray-finned fishes, which are distinguished by the structure of the paired fins that are supported by dermal rays. The class Actinopterygii comprises the subclasses Cladistia [Polypteriformes (bichirs…

The Joshua tree (Yucca brevifolia) is a remarkable evergreen, treelike plant found only in the southwestern United States. It was named by Mormon settlers in the early 1800s. Upon seeing the limbs of this plant branching upwards to heaven, the…

An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve…

A large, rabbit-sized rodent, belonging to the family Dasyproctidae and inhabiting South America and Central America, which is well adapted for speed because of its elongated hind legs. The agouti is a large rodent that resembles the rabbit or hare…

A model for the functioning of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the…

An order of actinopterygian fishes in the subdivision Elopomorpha, along with Elopiformes, Anguilliformes, and Saccopharyngiformes, all of which have a leptocephalous larval stage. Albuliformes, comprising three families, eight genera, and 30…

An order of the class Anthozoa (phylum Cnidaria), containing 2500–3000 species, commonly referred to as soft corals, sea fans, sea whips, and precious coral. Members of the Alcyonacea are strictly marine. They occur from northern to southern polar…

A large aquatic reptile of the family Alligatoridae. Alligators belong to the family Alligatoridae in the reptilian order Crocodylia (Crocodilia). Common usage generally restricts the name alligator to the two extant species of the genus Alligator.…

One of the four subclasses of Mammalia, containing a single order, the Multituberculata. The Allotheria first appeared in the Late Jurassic and survived well into the Cenozoic, a period of at least 100,000,000 years. Fossils are known from North…

Vicugna pacos, a member of the camel family (Camelidae), which has been domesticated for more than 2000 years and is economically important for its long, fine wool. The alpaca (Vicugna pacos; formerly, Lama pacos) [see illustration] belongs to the…

Plant growth forms characteristic of upper reaches of forests on mountain slopes. In such an environment, trees undergo gradual changes that, though subtle at first, may become dramatic beyond the dense forest as the zone of transition leads into…

Intergrading regions on mountain slopes characterized by specific plant life forms or species composition, and determined by complex environmental gradients.Along an altitudinal transect of a mountain, there are sequential changes in the physiognomy…

A fatty substance formed in the intestinal tract of the sperm whale (Physeter catodon). There is a question as to whether the origin of ambergris is normal or pathological, but it does serve as protection from the horny indigestible portions of the…

The tailless whip scorpions or whip spiders, an order of the class Arachnida. There are about 80 species in the tropics and subtropics. They are flattened, red to brown, and range from 5 to 45 mm (0.2 to 1.8 in.) in body length. All are nocturnal,…

Any protozoan moving by means of protoplasmic flow. In their entirety, the ameboid protozoa include naked amebas, those enclosed within a shell or test, as well as more highly developed representatives such as the heliozoians, radiolarians, and…

An order of actinopterygian fishes in the subclass Neopterygii. Amiiformes plus several related fossil orders comprise the Halecomorphi, a well-developed group known from the middle Mesozoic. The Amiiformes comprise several families that persisted…

A collective term for the classes Reptilia (reptiles), Aves (birds), and Mammalia (mammals) of the subphylum Vertebrata. The remaining vertebrates, including the several classes of fishes and the amphibians, are grouped together as the Anamnia.…

An order of Lobosia without protective coverings (tests). These protozoa range in size from about 4 micrometers to 0.08–0.12 in. (2–3 mm). Pellicles may be thin, as in Amoeba proteus, or thicker and less flexible, as in Thecamoeba verrucosa (Fig.…

One of four classes (the others are Reptilia, Aves, and Mammalia) composing the superclass Tetrapoda of the subphylum Vertebrata. The living amphibians number over 4700 species and are classified in three orders: Salientia or Anura (frogs and toads,…

Collectively, members of the class Amphibia are facing massive reductions in total numbers, and the overall biodiversity of amphibians across the globe is under threat. Scientists have determined that global climate change and habitat destruction…

A collective term, also referred to as Anamniota, for the vertebrate animals that lack an amnion in development, including the Agnatha, Chondrichthyes, Osteichthyes, and Amphibia. The amnion is a protective embryonic envelope that encloses the…

Formerly, a group of reptiles that was recognized in rank-based classifications. Amniotes were once divided into subclasses, based on the patterns of temporal fenestrae (openings in the skull roof posterior to the openings for the eyes). Subclass…

An extant order of the crustacean superorder Syncarida confined to freshwater and recorded from both surface and subterranean waters. Members of the order Anaspidacea (anaspidaceans) lack a carapace. The first thoracomere (thoracic somite) is…

The true eels, a large order of actinopterygian fishes, also known as the Apodes. The order Anguilliformes is related to Saccopharyngiformes (sackpharynx fishes), Elopiformes (tarpons), and Notacanthiformes (spiny eels and halosaurs); all members…

A method used by animals to conceal themselves from an enemy by appearing to be a section of the natural background. Animal camouflage is a strategy that animal organisms use to avoid or deflect the attacks of predators by misleading the latter's…

An animal's movement through the atmosphere sustained by aerodynamic reaction or other forces. Animal flight includes gliding and flapping flight. Gliding flight does not involve the generation of thrust, whereas flapping flight does generate thrust…

The classification rank comprising the animals. Animals are taxonomically assigned to the classification kingdom known as Animalia. (The other kingdoms for classification of organisms include Plantae, Fungi, Protista, and Monera; in certain…

The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta…

An order of branchiopod crustaceans, known as fairy shrimps and, in some cases, brine shrimps (Artemia). Members of the order Anostraca (anostracans) range up to about 100 mm (4 in.) in length but usually are much smaller. The trunk consists of…

An order of birds comprising two families, the screamers (Anhimidae) of South America and the worldwide waterfowl (Anatidae). They are closely related to the Galliformes (fowls), with the screamers being a rather intermediate group. The waterfowl…

Any of several living mammals in the suborder Vermilingua in the order Pilosa that feed on ants and termites. The common name of anteater is assigned to four species of living mammals classified in the suborder Vermilingua in the order Pilosa…

The name given to a group of African and Asian hoofed ruminants of the order Artiodactyla. The 91 species in 31 genera are divided into five subfamilies (Bovinae, Cephalophinae, Hippotraginae, Antilopinae, and Caprinae) in the family Bovidae. The…

A class of phylum Cnidaria (formerly known as Coelenterata) whose members are marine, live exclusively as polyps, and occur as solitary individuals or in clonal or colonial groups. Most adult anthozoans are attached to a firm object or the sea…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

An order of cnidarians (subclass Hexacorallia) commonly known as black, or thorny, corals. Antipatharians have been identified in all of the world's oceans at depths of 4 to 8600 m (13 to 28,200 ft), but most known species live between 100 and 1000…

One of the three living orders of the class Amphibia, encompassing the frogs and toads. Because members of the order Anura (anurans) lack a tail, they differ most obviously from salamanders (order Urodela; also called Caudata). Usually, frogs and…

Any of the vermiform (wormlike) marine mollusks covered by a cuticle invested with innumerable calcareous scales or spines (sclerites) forming a scleritome. There are two groups that together are known as the Aplacophora: (1) Chaetodermomorpha or…

The smallest order of the class Amphibia, consisting of the wormlike caecilians. The order Apoda is sometimes called Gymnophiona. Its members are known commonly as caecilians, which are wormlike, legless amphibians with indistinct or even hidden…

An order of birds consisting of two dissimilar groups, the swifts (Apodi) and the hummingbirds (Trochili). These birds have been placed together because of several anatomical specializations of the wings and feathers, although an alternative…

A little-studied group of ciliates comprising an order of the Holotrichia. The majority occur as commensals on marine crustaceans. This association is generally more intimate and complex than that of the chonotrichs found on related hosts. Their…

A class of marine, planktonic animals in the subphylum Tunicata. The class Appendicularia (alternatively known as Larvacea) is characterized by the persistence of a tail, notochord, gill slits, and dorsal nerve cord throughout life and by a unique…

The application of ecological principles to the solution of human problems and the maintenance of a quality life. It is assumed that humans are an integral part of ecological systems and that they depend upon healthy, well-operating, and productive…

The cultivation of freshwater and marine species for human purposes. Aquaculture, or aquatic farming, is concerned with the raising of animals and the harvesting of plants in freshwater and marine environments (Fig. 1). In other words, it is the…

The largest class of the subphylum Chelicerata in the phylum Arthropoda. Most familiar of the included orders in the class Arachnida are the spiders (Fig. 1), scorpions (Fig. 2), harvestmen, and mites and ticks. Members of the class Arachnida…

An order of nematodes in which the amphids are simple spirals that appear as elongate loops, shepherd's crooks, question marks, or circular forms. The cephalic sensilla are often separated into three circlets: the first two are papilliform or the…

A natural order of the class Arachnida, commonly known as the spiders. Members of the order Araneae (also called Araneida)—the spiders—are widespread over most of the land areas of the world and are well adapted to many different habitats. These…

An order of regular sea urchins (class Echinoidea) that have a stirodont Aristotle's lantern (jaw apparatus) with the epiphyses not meeting at the top of the lantern, and with keeled teeth that are T-shaped in cross section. The ambulacral plates of…

An order of Lobosia. The shell (test) of these protozoa has a single, well-defined aperture through which slender fingerlike pseudopodia (lobopodia) can be extended. The test often has an internal chitinous layer known as a glycocalyx and an…

An extinct group of mainly Lower Cambrian marine sponges which, although lacking spicules, possessed an intricate, highly porous skeleton of calcite. It was probably a monophyletic group; that is, all representatives were derived from a single…

An order of ancestrally wingless insects, commonly known as bristletails, in the subclass Apterygota; about 350 species are known in the two families, Machilidae and Meinertellidae. All have slender, spindle-shaped bodies of about 0.4–0.8 in. (1–2…

A class of the phylum Acanthocephala. The adult worms are parasitic in terrestrial vertebrates. The body wall and lemnisci of species in this class have numerous amitotically fragmented nuclei or a few ameboid giant nuclei. The main trunks of the…

A diversified group of diapsid reptiles, comprising dinosaurs (and their descendants, birds), crocodylians, pterosaurs (flying reptiles), and various forms closely related to these groups. In the past, the primitive archosaurs from the Triassic…

An order of teleost fishes, comprising the argentines or herring smelts, in the superorder Protacanthopterygii. Members of the order Argentiniformes are marine, deep-water fishes. They are identifiable by the following combination of characters: a…

A suborder of leeches (order Hirudinida) characterized by the absence of a protrusible proboscis and including both blood-feeding and macrophagous (predatory) varieties. Most notorious among the blood-feeding leeches in the Arhynchobdellida is the…

A heavily armored mammal in the order Cingulata. The order Cingulata (formerly Edentata or Xenarthra) comprises the armadillos, which are divided into two extant families: Chlamyphoridae and Dasypodidae. These creatures are characterized by bony…

The largest phylum in the animal kingdom, including the well-known insects, spiders, ticks, and crustaceans, as well as many smaller, less well-known groups. Members of the phylum Arthropoda, that is, arthropods, are the dominant metazoans…

The only surviving subclass of the echinoderm class Crinoidea. The Articulata include all living members of the echinoderm class Crinoidea and are generally regarded as the post-Paleozoic crinoids. Crinoids are exclusively marine organisms, as are…