Portal:Animals
Introduction
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft) and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.
Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing the echinoderms and chordates (including the vertebrates). Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.
Selected general articles
Kinorhyncha (Ancient Greek: κινέω, translit. kīnéō, lit. 'I move', ῥύγχος rhúnkhos "snout") is a phylum of small (1 mm or less) marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. Read more...
Osteichthyes (/ˌɒstiˈɪkθiiːz/), popularly referred to as the bony fish, is a diverse taxonomic group of fish that have skeletons primarily composed of bone tissue, as opposed to cartilage. The vast majority of fish are members of Osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, and over 435 families and 28,000 species. It is the largest class of vertebrates in existence today.
The group Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are about 420 million years old, which are also transitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.
Osteichthyes can be compared to Euteleostomi. In paleontology, the terms are synonymous. In ichthyology, the difference is that Euteleostomi presents a cladistic view which includes the terrestrial tetrapods that evolved from lobe-finned fish, whereas prior to 2014 the view of most ichthyologists was that Osteichthyes includes only fishes, and were therefore paraphyletic. However, in 2014, an ichthyology paper was published with phylogenetic trees that treat the Osteichthyes as a clade including tetrapods. Read more...
Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.
The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad. Read more...
The Asterozoa are a superclass in the phylum Echinodermata. Characteristics include a star-shaped body and radially divergent axes of symmetry. The superclass includes the two classes Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars, and the extinct order Somasteroidea. Read more...- Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. Read more...
Agnatha (Greek, ἀ-γνάθος "no jaws") is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. The group is sister to all vertebrates with jaws, known as gnathostomes.
Recent molecular data, both from rRNA and from mtDNA as well as embryological data strongly supports the hypothesis that living agnathans, the cyclostomes, are monophyletic. Read more...
Vertebrates /ˈvɜːrtɪbrɪts/ comprise all species of animals within the subphylum Vertebrata /-eɪ/ (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,276 species described. Vertebrates include the jawless fishes and jawed vertebrates, which include the cartilaginous fishes (sharks, rays, and ratfish) and the bony fishes.
Other vertebrates are the tetrapods, which include a bony fish clade known as the lobe-finned fishes, as well as amphibians, reptiles, birds and mammals. Read more...
Urechis caupo
The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now universally considered to represent derived annelid worms, which have lost their segmentation. Their presumed sister group is the Capitellidae. The majority of echiurans live in shallow water, but there are also deep sea forms. More than 230 species have been described.
Spoon worms fossilize poorly and the earliest known specimen is from the Upper Carboniferous (Pennsylvanian). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian. Read more...
Phylactolaemata is a class of the phylum Bryozoa whose members live only in freshwater environments. Like all bryozoans, they filter feed by means of an extensible "crown" of ciliated tentacles called a lophophore. They live in colonies, each of which consists of clones of the founding member. Unlike those of some marine bryozoans, phylactolaemate colonies consist of only one type of zooid, the feeding forms known as autozooids. These are supported by an unmineralized "exoskeleton" made of gelatinous material or protein, secreted by the zooids.
The class contains only one extant order, Plumatellida. Read more...
Octocorallia (also known Alcyonaria) is a subclass of Anthozoa comprising around 3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fans and sea whips) within three orders: Alcyonacea, Helioporacea, and Pennatulacea. These organisms have an internal skeleton secreted by mesoglea and polyps with eight tentacles and eight mesentaries. As with all Cnidarians these organisms have a complex life cycle including a motile phase when they are considered plankton and later characteristic sessile phase.
Octocorals have existed at least since the Ordovician as shown by Maurits Lindström's findings in the 1970s. Read more...
Air-breathing land gastropod Helix pomatia, the Roman snail
The gastropods (/ˈɡæstroʊpɒdz/), more commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca, called Gastropoda. This class comprises snails and slugs from saltwater, from freshwater, and from the land. There are many thousands of species of sea snails and slugs, as well as freshwater snails, freshwater limpets, and land snails and slugs.
The class Gastropoda contains a vast total of named species, second only to the insects in overall number. The fossil history of this class goes back to the Late Cambrian. , 721 families of gastropods are known, of which 245 are extinct and appear only in the fossil record, while 476 are currently extant with or without a fossil record. Read more...
Priapulida (priapulid worms, from Gr. πριάπος, priāpos 'Priapus' + Lat. -ul-, diminutive), sometimes referred to as penis worms, is a phylum of unsegmented marine worms. The name of the phylum relates to the Greek god of fertility, because their general shape and their extensible spiny introvert (eversible proboscis) may recall the shape of a penis. They live in the mud and in comparatively shallow waters up to 90 metres (300 ft) deep. Some species show a remarkable tolerance for hydrogen sulfide and anoxia. They can be quite abundant in some areas. In an Alaskan bay as many as 85 adult individuals of Priapulus caudatus per square meter has been recorded, while the density of its larvae can be as high as 58,000 per square meter.
Together with Echiura and Sipuncula, they were once placed in the taxon Gephyrea, but consistent morphological and molecular evidence supports their belonging to Ecdysozoa, which also includes arthropods and nematodes. Fossil findings show that the mouth design of the stem-arthropod Pambdelurion is identical with that of priapulids, indicating that their mouth is an original trait inherited from the last common ancestor of both priapulids and arthropods, even if modern arthropods no longer possess it. Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert (scalids). They feed on slow-moving invertebrates, such as polychaete worms. Read more...
Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size from 50 to 100 millimetres (2.0 to 3.9 in) long, reaching 2 metres in extreme cases, and 1 to 3 millimetres (0.039 to 0.118 in) in diameter. Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams, puddles, and cisterns. The adult worms are free-living, but the larvae are parasitic on arthropods, such as beetles, cockroaches, mantids, orthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots. Read more...
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma. Read more...
A shell of the scaphopod Antalis vulgaris from France
The tusk shells or tooth shells, often referred to by the more-technical term scaphopods /ˈskæfəˌpɒd/ (From Ancient Greek, σκᾰ́φης (skáphē) "boat", and πούς (poús) "foot"), are members of a class of shelled marine mollusc with worldwide distribution, and are the only class of exclusively infaunal marine molluscs. Shells of species within this class range from about 0.5 to 15 cm in length. Members of the order Dentaliida tend to be significantly larger than those of the order Gadilida.
These molluscs live in soft substrates offshore (usually not intertidally). Because of this subtidal habitat and the small size of most species, many beachcombers are unfamiliar with them; their shells are not as common or as easily visible in the beach drift as the shells of sea snails and clams. Read more...
The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a group containing 144–320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms. Sipuncula signifies "little tube or siphon". Traditionally considered a phylum, they might be a subgroup of phylum Annelida based on recent molecular work. Read more...
The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, pseudocoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine (except for two species), and the Chaetonotida, some of which are marine and some freshwater. Nearly eight hundred species of gastrotrich have been described.
Gastrotrichs have a simple body plan with a head region, with a brain and sensory organs, and a trunk with a simple gut and the reproductive organs. They have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, sucking up organic particles with their muscular pharynx. They are hermaphrodites, the marine species producing eggs which develop directly into miniature adults. The freshwater species are parthenogenetic, producing unfertilised eggs, and at least one species is viviparous. Gastrotrichs mature with great rapidity and have lifespans of only a few days. Read more...
Ctenophora (/tɪˈnɒfərə/; singular ctenophore, /ˈtɛnəfɔːr/ or /ˈtiːnəfɔːr/; from the Greek κτείς kteis "comb" and φέρω pherō "to carry"; commonly known as comb jellies) comprise a phylum of invertebrate animals that live in marine waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are the largest animals to swim with the help of cilia.
Depending on the species, adult ctenophores range from a few millimeters to 1.5 m (4 ft 11 in) in size. Only 100 to 150 species have been validated, and possibly another 25 have not been fully described and named. The textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("little tentacles") that are covered with colloblasts, sticky cells that capture prey. Their bodies consist of a mass of jelly, with one layer two cells thick on the outside and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped cydippids with retractable tentacles that capture prey, the flat generally combless platyctenids, and the large-mouthed beroids, which prey on other ctenophores.
Almost all ctenophores function as predators, taking prey ranging from microscopic larvae and rotifers to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed. Read more...
The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, they do not have parapodia and their heads are less developed. Read more...
Vertebrates /ˈvɜːrtɪbrɪts/ comprise all species of animals within the subphylum Vertebrata /-eɪ/ (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,276 species described. Vertebrates include the jawless fishes and jawed vertebrates, which include the cartilaginous fishes (sharks, rays, and ratfish) and the bony fishes.
Other vertebrates are the tetrapods, which include a bony fish clade known as the lobe-finned fishes, as well as amphibians, reptiles, birds and mammals. Read more...
An Oroperipatus species
Onychophora (from Ancient Greek, onyches, "claws"; and pherein, "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, Peripatus), is a phylum of elongate, soft-bodied, many-legged panarthropods. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. They prey upon smaller animals such as insects, which they catch by squirting an adhesive slime.
Approximately 200 species of velvet worms have been described, although the true number of species is likely greater. The two extant families of velvet worms are Peripatidae and Peripatopsidae. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are all found south of the equator. It is the only phylum within animalia that is wholly endemic to terrestrial environments. Velvet worms are considered close relatives of the Arthropoda and Tardigrada, with which they form the taxon Panarthropoda. This makes them of palaeontological interest, as they can help reconstruct the ancestral arthropod. In modern zoology, they are particularly renowned for their curious mating behaviour and for bearing live young. Read more...
Great white shark, Carcharodon carcharias
Chondrichthyes (/kɒnˈdrɪkθiiːz/; from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') is a class that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).
Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates. Read more...
The lancelets, also known as amphioxi (singular, amphioxus), consist of about 32 species of fish-like marine chordates in the order Amphioxiformes. They are distributed in shallow and coastal temperate (as far north as Norway), subtropical and tropical seas around the world, usually found half-buried in sand. The only exception is Asymmetron inferum, a species known from the vicinity of whale falls at a depth of about 225 m (738 ft). In Asia, lancelets are harvested commercially as food for humans and domesticated animals.
They are the modern representatives of the subphylum Cephalochordata, formerly thought to be the sister group of the craniates. They are an important object of study in zoology as they provide indications about the evolutionary origins of the vertebrates (to which they do not belong): lancelets split from vertebrates more than 520 million years ago and are regarded as similar to the archetypal vertebrate form. Their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions. The genome of a few species has been sequenced, starting with Branchiostoma floridae in 2008. Read more...
Sponges, the members of the phylum Porifera (/pəˈrɪfərə/; meaning "pore bearer"), are a basal Metazoa (animal) clade as a sister of the Diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. The branch of zoology that studies sponges is known as spongiology.
Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Sponges were first to branch off the evolutionary tree from the common ancestor of all animals, making them the sister group of all other animals. Read more...
Clockwise from above left: Green sea turtle (Chelonia mydas), Tuatara (Sphenodon punctatus), Nile crocodile (Crocodylus niloticus), and Sinai agama (Pseudotrapelus sinaitus).
Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. The study of these traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.
Because some reptiles are more closely related to birds than they are to other reptiles (e.g., crocodiles are more closely related to birds than they are to lizards), the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade (consisting of all descendants of a common ancestor). For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living Diapsids. Read more...- Symbion is the name of a genus of aquatic animals, less than 0.5 mm wide, found living attached to the bodies of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983. Read more...
Mammals (/məˈmeɪliə/ from Latin mamma "breast") are vertebrate animals constituting the class Mammalia, and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals. The largest orders are the rodents, bats and Soricomorpha (shrews and others). The next three are the Primates (humans, apes, monkeys, and others), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and others).
In cladistics, which reflect evolution, mammals are classified as endothermic amniotes. They are the only living Synapsida, which together with the Sauropsida (reptiles and birds) form the Amniota clade. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been among the dominant terrestrial animal groups from 66 million years ago to the present. Read more...
Tardigrades (/ˈtɑːrdɪˌɡreɪd/; also known colloquially as water bears, or moss piglets) are a phylum of water-dwelling, eight-legged, segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who gave them the name of "little water bears". The name Tardigrada (meaning "slow steppers") was given three years later by the Italian biologist Lazzaro Spallanzani. They have been found everywhere: from mountaintops to the deep sea and mud volcanoes; from tropical rain forests to the Antarctic. Tardigrades are among the most resilient known animals, with individual species able to survive extreme conditions that would be rapidly fatal to nearly all other known life forms, such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation. Tardigrades have even survived exposure to outer space. About 1,150 known species form the phylum Tardigrada, a part of the superphylum Ecdysozoa. The group includes fossils dating from 530 million years ago, in the Cambrian period.
Usually, tardigrades are about 0.5 mm (0.02 in) long when they are fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or sucking disks. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very low-power microscope, making them accessible to students and amateur scientists. Read more...- Not to be confused with Enopla, a clade of nemertean worms.
Enoplea (enopleans) is a class, which with the classes Secernentea and Chromadorea make up the phylum Nematoda in current taxonomy. The Enoplea are considered to be a more ancestral group than the Chromadorea, and researchers have referred to its members as the "ancestrally diverged nematodes", compared to the "more recently diverged nematodes" of Chromadorea. Read more... 
Bedford's flatworm, Pseudobiceros bedfordi
The flatworms, flat worms, Platyhelminthes, Plathelminthes, or platyhelminths (from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning "worm") are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.
In traditional medicinal texts, Platyhelminthes are divided into Turbellaria, which are mostly non-parasitic animals such as planarians, and three entirely parasitic groups: Cestoda, Trematoda and Monogenea; however, since the turbellarians have since been proven not to be monophyletic, this classification is now deprecated. Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments, such as leaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers of hermaphroditic, segment-like proglottids that detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infesting aquatic animals, and their larvae metamorphose into the adult form after attaching to a suitable host. Read more...
Sponges, the members of the phylum Porifera (/pəˈrɪfərə/; meaning "pore bearer"), are a basal Metazoa (animal) clade as a sister of the Diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. The branch of zoology that studies sponges is known as spongiology.
Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Sponges were first to branch off the evolutionary tree from the common ancestor of all animals, making them the sister group of all other animals. Read more...
By Johann Wilhelm Spengel, 1893
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. Their closest relatives are the echinoderms. There are 111 known species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii. The two families Harrimaniidae and Ptychoderidae separated at least 370 million years ago.
Until recently, it was thought that all species lived in the sediment on the seabed, subsisting as deposit feeders or suspension feeders. However, the last decade has seen the description of a new family, the Torquaratoridae, evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites. It is assumed that the ancestors of acorn worms used to live in tubes like their relatives Pterobranchia, but that they eventually started to live a safer and more sheltered existence in sediment burrows instead. Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8 ft 2 in), although most acorn worms are much smaller. Due to secretions containing elements like iodine, the animals have an iodoform-like smell. Read more...
Hexacorallia is a subclass of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are colonial and reef-forming, as well as all sea anemones, and zoanthids, arranged within five extant orders. The hexacorallia are distinguished from another subclass of Anthozoa, Octocorallia, in having six or fewer axes of symmetry in their body structure; the tentacles are simple and unbranched and normally number more than eight. These organisms are formed of individual soft polyps which in some species live in colonies and can secrete a calcite skeleton. As with all Cnidarians, these organisms have a complex life cycle including a motile planktonic phase and a later characteristic sessile phase. Hexacorallia also include the significant extinct order of rugose corals. Read more...
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains over 16,000 species, most of which are terrestrial. Although their name suggests they have myriad (10,000) legs, myriapods range from having up to 750 legs (the millipede Illacme plenipes) to having fewer than ten legs.
The fossil record of myriapods reaches back into the late Silurian, although molecular evidence suggests a diversification in the Cambrian Period, and Cambrian fossils exist which resemble myriapods. The oldest unequivocal myriapod fossil is of the millipede Pneumodesmus newmani, from the late Silurian (428 million years ago). P. newmani is also important as the earliest known terrestrial animal. The phylogenetic classification of myriapods is still debated. Read more...
A collection of modern and extinct chelicerates. Clockwise from top left: a sea spider, Pentecopterus (an extinct eurypterid), a spiny orb-weaver and an atlantic horseshoe crab.
The subphylum Chelicerata (New Latin, from French chélicère, from Greek χήλή, khēlē "claw, chela" and κέρας, kéras "horn") constitutes one of the major subdivisions of the phylum Arthropoda. It contains the horseshoe crabs, sea spiders, arachnids (including scorpions and spiders) and several extinct lineages, such as the eurypterids.
The Chelicerata originated as marine animals in the Middle Cambrian period; the first confirmed chelicerate fossils, belonging to Sanctacaris, date from 508 million years ago. The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are indeed chelicerates. On the other hand, there are over 77,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species. Read more...
Pterobranchia is a clade of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group.
The class Pterobranchia was established by Ray Lankester in 1877. It contained, at that time, the single genus Rhabdopleura. Rhabdopleura was at first regarded as an aberrant polyzoon, but when the Challenger report on Cephalodiscus was published in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta. Read more...
The nematodes (UK: /ˈnɛmətoʊdz/, US: /ˈniːməˌtoʊdz/) or roundworms constitute the phylum Nematoda (also called Nemathelminthes). They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, and unlike flatworms, have tubular digestive systems with openings at both ends.
Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode, a claim which has since been repeated in numerous publications, without additional investigation, in an attempt to accentuate the importance and ubiquity of nematodes in the global ecosystem (rather than as a sign of agreement with the estimated taxonomic figure). Many other publications have since vigorously refuted this claim on the grounds that it is unsupported by fact, and is the result of speculation and sensationalism. More recent, fact-based estimates have placed the true figure closer to 40,000 species worldwide. Read more...
Monogeneans are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female reproductive structures. Monogeneans have a series of hooks which are used to attach onto fish, and as a result, could lead to infections.
Some monogeneans are oviparous (egg-laying) and some are viviparous (live-bearing). Oviparous varieties release eggs into the water. Viviparous varieties release larvae, which immediately attach to another host. The genus Gyrodactylus is an example of a viviparous variety, while the genus Dactylogyrus is an example of an oviparous variety. Read more...
Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. With the exception of two acoel freshwater species, all known Acoelomorphs are marine. Read more...
Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres (0.020 in) long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches, and others are found in polar waters. One class lives only in a variety of freshwater environments, and a few members of a mostly marine class prefer brackish water. Over 4,000 living species are known. One genus is solitary and the rest are colonial.
The phylum was originally called "Polyzoa", but this term was superseded by "Bryozoa" in 1831. Another group of animals discovered subsequently, whose filtering mechanism looked similar, was also included in "Bryozoa" until 1869, when the two groups were noted to be very different internally. The more recently discovered group was given the name Entoprocta, while the original "Bryozoa" were called "Ectoprocta". However, "Bryozoa" has remained the more widely used term for the latter group. Read more...
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. Modern amphibians are all Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this. The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. They are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia. Read more...
A live lined chiton, Tonicella lineata photographed in situ: The anterior end of the animal is to the right.
Chitons /ˈkaɪtənz/ are marine molluscs of varying size in the class Polyplacophora, formerly known as Amphineura. About 940 extant and 430 fossil species are recognized.
They are also sometimes known as sea cradles or "coat-of-mail shells", or more formally as loricates, polyplacophorans, and occasionally as polyplacophores. Read more...
Birds, also known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the world's most numerically-successful class of tetrapods, with approximately ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites, penguins, and diverse endemic island species of birds. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.
Reverse genetic engineering and the fossil record both demonstrate that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs. The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics. Read more...
The Acoela or acoels (from Greek: άκοιλα (akoila), a + coel i.e. without a cavity) are a class of small and simple invertebrates in the phylum Xenacoelomorpha, a basal Bilaterian group, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms. Read more...
Phoronids (scientific name Phoronida, sometimes called horseshoe worms) are a small phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long.
The bottom end of the body is an ampulla (a flask-like swelling), which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, cilia (little hairs) on the sides of the tentacles draw food particles to the mouth, which is inside and slightly to one side of the base of the lophophore. Unwanted material can be excluded by closing a lid above the mouth or be rejected by the tentacles, whose cilia can switch into reverse. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore. Read more...
Echinoderm is the common name given to any member of the phylum Echinodermata (from Ancient Greek, ἐχῖνος, echinos – "hedgehog" and δέρμα, derma – "skin") of marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as sea stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone.
The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.
Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian. One group of Cambrian echinoderms, the cinctans (Homalozoa), which are close to the base of the echinoderm origin, have been found to possess external gills used for filter feeding, like chordata and hemichordata. Read more...
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Did you know...
- ... that players take control of animals to solve puzzles and defeat enemies in Space Station Silicon Valley?
- ... that a verse from Psalm 36 is seen as endorsing animal welfare and animal rights?
- ... that the philosopher John Hadley argues that the territory of wild animals should be considered their property?
- ... that Anna L. Peterson argues that the usual separation of animal and environmental ethics is based on mistaken conceptions of nature, humans, animals, and the relationships among them?
- ... that an animal called the "Seaweed of Death from Hana" has caused poisonings of aquarium hobbyists?
- ... that songs and books for children often depict happy farm animals in attractive countryside, glossing over the realities of impersonal, mechanized activities involved in modern intensive farming?
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Selected images
The bilaterian gut develops in two ways. In many protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus.
Artistic vision: Still Life with Lobster and Oysters by Alexander Coosemans, c. 1660
Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other organisms.
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Idealised bilaterian body plan. With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.
Dickinsonia costata from the Ediacaran biota (c. 635–542 MYA) is one of the earliest animal species known.
Spiral cleavage in a sea snail embryo
Sexual reproduction is nearly universal in animals, such as these dragonflies.
Sides of beef in a slaughterhouse
Non-bilaterians include sponges (centre) and corals (background).
The blue whale is the largest animal that has ever lived.
Ecdysis: a dragonfly has emerged from its dry exuviae and is expanding its wings. Like other arthropods, its body is divided into segments.
Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla by 1809.
Hydrothermal vent mussels and shrimps
A gun dog retrieving a duck during a hunt
Anomalocaris canadensis is one of the many animal species that emerged in the Cambrian explosion, starting some 542 million years ago, and found in the fossil beds of the Burgess shale.
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