Over 1.5 million
living animal
species have been
described, of which around 1.05 million are
insects, over 85,000 are
molluscs, and around 65,000 are
vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex
ecologies and
interactions with each other and their environments, forming intricate
food webs. The scientific study of animals is known as
zoology, and the study of animal behaviors is known as
ethology.
Entries here consist of
Good and
Featured articles, which meet a core set of high editorial standards.
The European hare (Lepus europaeus), also known as the brown hare, is a
species of
hare native to
Europe and parts of
Asia. It is among the largest hare species and is adapted to temperate, open country. Hares are
herbivorous and feed mainly on grasses and herbs, supplementing these with twigs, buds, bark and field crops, particularly in winter. Their natural
predators include large
birds of prey,
canids and
felids. They rely on high-speed endurance running to escape predation, having long, powerful limbs and large nostrils.
Generally nocturnal and shy in nature, hares change their behaviour in the spring, when they can be seen in broad daylight chasing one another around in fields. During this spring frenzy, they sometimes strike one another with their paws ("boxing"). This is usually not competition between males, but a female hitting a male, either to show she is not yet ready to mate or to test his determination. The female nests in a depression on the surface of the ground rather than in a
burrow and the young are active as soon as they are born. Litters may consist of three or four young and a female can bear three litters a year, with hares living for up to twelve years. The breeding season lasts from January to August. (Full article...)
Anatomical diagram of an adult female
chambered nautilus, the best known species of nautilus, a "
living fossil" related to the octopuses. The animal has a primitive brain that forms a ring around its
oesophagus, has four gills (all other
cephalopods have only two), and can only move shell-first (seemingly "backwards") by pumping water out through its funnel. The shell and tentacles are shown here as shadows.
The coconut octopus (Amphioctopus marginatus) is a medium-sized
cephalopod found in tropical waters of the western Pacific Ocean. It commonly preys upon
shrimp,
crabs, and
clams, and displays unusual behaviour, including bipedal walking and gathering and using
coconut shells and seashells for shelter.
A soybean cyst nematode (Heterodera glycines), together with an egg, as viewed through a low-temperature
scanning electron microscope at 1000x magnification. This
nematode infects the roots of soybeans, and the female nematode eventually becomes a
cyst. Infection causes various symptoms that may include
chlorosis of the leaves and stems, root necrosis, loss in seed yield and suppression of root and shoot growth.
Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a
phylum of simple,
aquaticinvertebrate animals, nearly all living in sedentary
colonies. Typically about 0.5 millimetres (1⁄64 in) long, they have a special feeding structure called a
lophophore, a "crown" of tentacles used for
filter feeding. Most
marine bryozoans live in tropical waters, but a few are found in
oceanic trenches and polar waters. The bryozoans are classified as the
marine bryozoans (Stenolaemata),
freshwater bryozoans (Phylactolaemata), and
mostly-marine bryozoans (Gymnolaemata), a few members of which prefer
brackish water. 5,869living species are known. Originally all of the crown group Bryozoa were colonial, but as an adaptation to a mesopsammal (interstitial spaces in marine sand) life or to deep‐sea habitats, secondarily solitary forms have since evolved. Solitary species has been described in four
genera; Aethozooides, Aethozoon, Franzenella and Monobryozoon). The latter having a statocyst‐like organ with a supposed excretory function. (Full article...)
Brittle stars, serpent stars, or ophiuroids (from
Latin ophiurus 'brittle star'; from
Ancient Greekὄφις (óphis) 'serpent', and οὐρά (ourá) 'tail'; referring to the serpent-like arms of the brittle star) are
echinoderms in the class Ophiuroidea, closely related to
starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens. (Full article...)
The flatworms, flat worms, Platyhelminthes, 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. Being
acoelomates (having no
body cavity), and having no specialised
circulatory and
respiratoryorgans, they are restricted 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 can not be processed continuously. (Full article...)
The rock hyrax (Procavia capensis), also known as the dassie, is one of four living species of the order
Hyracoidea, and the only living species in its genus. Like all
hyraxes, it is a medium-sized terrestrial mammal between 4 kilograms (9 lb) and 5 kilograms (11 lb) in mass, with short ears and tail. The rock hyrax is found across Africa and the Middle East, at elevations up to 4,200 metres (13,800 ft). It resides in habitats with rock crevices which it uses to escape from predators. Along with the other hyrax species and the
manatee, these are the animals most closely related to the elephant.
The brown pelican (Pelecanus occidentalis) is a bird of the pelican family,
Pelecanidae, one of three species found in the Americas and one of two that feed by diving into water. This photograph shows a juvenile brown pelican gliding over the Pacific Ocean at
Bodega Head, California.
After hatching, the pelican chicks are fed on
regurgitated predigested fish and take about two months to fledge. When they leave the nest, they are at first unable to fly and take wing several weeks later. When the parents cease to feed them, some six months later, each will have consumed around 70 kg (150 lb) of fish. The juvenile brown pelican does not acquire adult plumage until three years of age, when the feathers on the neck become paler, the upperparts striped, the wing feathers grayer, and the belly acquires dark spots.
Haliotis laevigata is a species of marine
mollusc in the family
Haliotidae,
endemic to Tasmania and the southern and western coasts of Australia. This picture shows five views of a green H. laevigata shell, 7.5 centimetres (3.0 in) in length. The holes in the shell, characteristic of
abalones, are respiratory apertures for venting water from the gills and for releasing sperm and eggs into the
water column.
A
macro view of a Gonia capitatafly feeding on
honey, showing its
proboscis and
pedipalps (the two appendages protruding from the proboscis), two types of insect mouthparts. The proboscis actually comprises the labium, a quadrupedal structure, and a sponge-like labellum at the end. Flies eat solid food by secreting
saliva and dabbing it over the food item. As the saliva dissolves the food, the solution is then drawn up into the mouth as a liquid. The labellum's surface is covered by minute food channels which form a tube leading to the
esophagus, and food is drawn up the channels by
capillary action.
The maxima clam (Tridacna maxima) is a species of
bivalve found throughout the
Indo-Pacific. It is found on the surface of reefs or sand, or partly embedded in coral (as with this specimen), in the oceans surrounding east Africa, India, China, Australia, Southeast Asia and the islands of the Pacific. This clam is much sought after in the aquarium trade, as its often striking coloration—the result of crystalline pigment—mimics that of the true
giant clam.
A cross section of a post-
clitellum segment of an annelid (ringed worm); almost all segments of an annelid contain the same set of organs and parts, a pattern called
metamerism. Annelids have no lungs, but rather exchange
carbon dioxide and
oxygen directly through the moist skin when blood reaches the extremely fine capillaries of the body walls; a dry worm cannot breathe and will die of suffocation. The worm's red blood, which does not consist of
platelets or
red cells but mostly of a liquid containing suspended
hemoglobin, makes a circuit up and down the animal in its closed circulatory systems.
The paddyfield pipit (Anthus rufulus) is a
passerine bird in the family
Motacillidae, comprising
pipits,
longclaws and
wagtails. About 15 cm (6 in) in length and native to southern Asia, its plumage in both sexes is greyish-brown above and paler yellowish-brown below, with dark streaking on the breast. A bird of open country, pasture and cultivated fields, it sometimes makes short flights, but mostly runs on the ground, foraging for insects and other small invertebrates. The paddyfield pipit builds its cup-shaped nest in a concealed location on the ground, and may have two or more broods in a year. This A. r. rufulus individual was photographed in
Kanha Tiger Reserve, in the Indian state of
Madhya Pradesh.
Glaucus atlanticus is a species of small, blue
sea slug. This
pelagic aeolid
nudibranch floats upside down, using the surface tension of the water to stay up, and is carried along by the winds and ocean currents. The blue side of their body faces upwards, blending in with the blue of the water, while the grey side faces downwards, blending in with the silvery surface of the sea. G. atlanticus feeds on other pelagic creatures, including the
Portuguese man o' war.
Entries here consist of
Good and
Featured articles, which meet a core set of high editorial standards.
Mephitidae is a
family of
mammals in the
orderCarnivora, which comprises the
skunks and
stink badgers. A member of this family is called a mephitid. The skunks of the family are widespread across the Americas, while the stink badgers are in the
Greater Sunda Islands of southeast Asia. Species inhabit a variety of habitats, though typically grassland, forest, and shrubland. Most mephitids are 20–50 cm (8–20 in) long, plus a 10–40 cm (4–16 in) tail, though the
pygmy spotted skunk can be as small as 11 cm (4 in) plus a 7 cm (3 in) tail, and some
striped skunks can be up to 82 cm (32 in) plus a 40 cm (16 in) tail. No estimates have been made for overall population sizes of any of the species, but two species are classified as
vulnerable. Mephetids in general are not domesticated, though skunks are sometimes
kept as pets.
The twelve species of Mephitidae are split into four
genera: the
monotypicConepatus, hog-nosed skunks; Mephitis, skunks; Mydaus, stink badgers; and Spilogale, spotted skunks. Mephitidae was traditionally a
clade within the
Mustelidae family, with the stink badgers combined with other
badgers within the Melinae genus, but more recent genetic evidence resulted in the consensus to separate Mephitidae into its own family. Extinct species have also been placed into all of the extant genera besides Mydaus, as well as 9 extinct genera; 26 extinct Mephitidae species have been found, though due to ongoing research and discoveries the exact number and categorization is not fixed. (Full article...)
...that Caecillians, an order of
amphibians resembling snakes or worms, possess a pair of tentacles, located between their eyes and nostrils which are probably used for
olfaction?
Image 2A brilliantly-coloured
oriental sweetlips fish (Plectorhinchus vittatus) waits while two boldly-patterned
cleaner wrasse (Labroides dimidiatus) pick parasites from its skin. The spotted tail and fin pattern of the sweetlips signals sexual maturity; the behaviour and pattern of the
cleaner fish signal their availability for
cleaning service, rather than as prey (from Animal coloration)
Image 3Non-bilaterians include sponges (centre) and corals (background). (from Animal)
Image 31The red
pigment in a flamingo's plumage comes from its diet of shrimps, which get it from microscopic algae. (from Animal coloration)
Image 32A
praying mantis in
deimatic or threat pose displays conspicuous patches of colour to startle potential predators. This is not warning coloration as the insect is palatable. (from Animal coloration)
Image 33Simplified schematic of an island's fauna – all its animal species, highlighted in boxes (from Fauna)
Image 34Animal anatomical engraving from Handbuch der Anatomie der Tiere für Künstler. (from Zoology)
Image 37Idealised
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. (from Animal)
Image 38The bilaterian gut develops in two ways. In many
protostomes, the blastopore develops into the mouth, while in
deuterostomes it becomes the anus. (from Animal)
Image 44The microscopic cave snail Zospeum tholussum, found at depths of 743 to 1,392 m (2,438 to 4,567 ft) in the
Lukina Jama–Trojama cave system of
Croatia, is completely blind with a translucent shell (from Fauna)
The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species,[1] along with their principal habitats (terrestrial, fresh water,[2] and marine),[3] and free-living or parasitic ways of life.[4] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[5] Using patterns within the
taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[6][7][a]
^The application of
DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000
insect species for
Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (
Cecidomyiidae).[8]
^Stork, Nigel E. (January 2018). "How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth?". Annual Review of Entomology. 63 (1): 31–45.
doi:
10.1146/annurev-ento-020117-043348.
PMID28938083.
S2CID23755007. Stork notes that 1m insects have been named, making much larger predicted estimates.
^
abcdNicol, David (June 1969). "The Number of Living Species of Molluscs". Systematic Zoology. 18 (2): 251–254.
doi:
10.2307/2412618.
JSTOR2412618.
^Sluys, R. (1999). "Global diversity of land planarians (Platyhelminthes, Tricladida, Terricola): a new indicator-taxon in biodiversity and conservation studies". Biodiversity and Conservation. 8 (12): 1663–1681.
doi:
10.1023/A:1008994925673.
S2CID38784755.
Over 1.5 million
living animal
species have been
described, of which around 1.05 million are
insects, over 85,000 are
molluscs, and around 65,000 are
vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex
ecologies and
interactions with each other and their environments, forming intricate
food webs. The scientific study of animals is known as
zoology, and the study of animal behaviors is known as
ethology.
Entries here consist of
Good and
Featured articles, which meet a core set of high editorial standards.
The European hare (Lepus europaeus), also known as the brown hare, is a
species of
hare native to
Europe and parts of
Asia. It is among the largest hare species and is adapted to temperate, open country. Hares are
herbivorous and feed mainly on grasses and herbs, supplementing these with twigs, buds, bark and field crops, particularly in winter. Their natural
predators include large
birds of prey,
canids and
felids. They rely on high-speed endurance running to escape predation, having long, powerful limbs and large nostrils.
Generally nocturnal and shy in nature, hares change their behaviour in the spring, when they can be seen in broad daylight chasing one another around in fields. During this spring frenzy, they sometimes strike one another with their paws ("boxing"). This is usually not competition between males, but a female hitting a male, either to show she is not yet ready to mate or to test his determination. The female nests in a depression on the surface of the ground rather than in a
burrow and the young are active as soon as they are born. Litters may consist of three or four young and a female can bear three litters a year, with hares living for up to twelve years. The breeding season lasts from January to August. (Full article...)
Anatomical diagram of an adult female
chambered nautilus, the best known species of nautilus, a "
living fossil" related to the octopuses. The animal has a primitive brain that forms a ring around its
oesophagus, has four gills (all other
cephalopods have only two), and can only move shell-first (seemingly "backwards") by pumping water out through its funnel. The shell and tentacles are shown here as shadows.
The coconut octopus (Amphioctopus marginatus) is a medium-sized
cephalopod found in tropical waters of the western Pacific Ocean. It commonly preys upon
shrimp,
crabs, and
clams, and displays unusual behaviour, including bipedal walking and gathering and using
coconut shells and seashells for shelter.
A soybean cyst nematode (Heterodera glycines), together with an egg, as viewed through a low-temperature
scanning electron microscope at 1000x magnification. This
nematode infects the roots of soybeans, and the female nematode eventually becomes a
cyst. Infection causes various symptoms that may include
chlorosis of the leaves and stems, root necrosis, loss in seed yield and suppression of root and shoot growth.
Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a
phylum of simple,
aquaticinvertebrate animals, nearly all living in sedentary
colonies. Typically about 0.5 millimetres (1⁄64 in) long, they have a special feeding structure called a
lophophore, a "crown" of tentacles used for
filter feeding. Most
marine bryozoans live in tropical waters, but a few are found in
oceanic trenches and polar waters. The bryozoans are classified as the
marine bryozoans (Stenolaemata),
freshwater bryozoans (Phylactolaemata), and
mostly-marine bryozoans (Gymnolaemata), a few members of which prefer
brackish water. 5,869living species are known. Originally all of the crown group Bryozoa were colonial, but as an adaptation to a mesopsammal (interstitial spaces in marine sand) life or to deep‐sea habitats, secondarily solitary forms have since evolved. Solitary species has been described in four
genera; Aethozooides, Aethozoon, Franzenella and Monobryozoon). The latter having a statocyst‐like organ with a supposed excretory function. (Full article...)
Brittle stars, serpent stars, or ophiuroids (from
Latin ophiurus 'brittle star'; from
Ancient Greekὄφις (óphis) 'serpent', and οὐρά (ourá) 'tail'; referring to the serpent-like arms of the brittle star) are
echinoderms in the class Ophiuroidea, closely related to
starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens. (Full article...)
The flatworms, flat worms, Platyhelminthes, 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. Being
acoelomates (having no
body cavity), and having no specialised
circulatory and
respiratoryorgans, they are restricted 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 can not be processed continuously. (Full article...)
The rock hyrax (Procavia capensis), also known as the dassie, is one of four living species of the order
Hyracoidea, and the only living species in its genus. Like all
hyraxes, it is a medium-sized terrestrial mammal between 4 kilograms (9 lb) and 5 kilograms (11 lb) in mass, with short ears and tail. The rock hyrax is found across Africa and the Middle East, at elevations up to 4,200 metres (13,800 ft). It resides in habitats with rock crevices which it uses to escape from predators. Along with the other hyrax species and the
manatee, these are the animals most closely related to the elephant.
The brown pelican (Pelecanus occidentalis) is a bird of the pelican family,
Pelecanidae, one of three species found in the Americas and one of two that feed by diving into water. This photograph shows a juvenile brown pelican gliding over the Pacific Ocean at
Bodega Head, California.
After hatching, the pelican chicks are fed on
regurgitated predigested fish and take about two months to fledge. When they leave the nest, they are at first unable to fly and take wing several weeks later. When the parents cease to feed them, some six months later, each will have consumed around 70 kg (150 lb) of fish. The juvenile brown pelican does not acquire adult plumage until three years of age, when the feathers on the neck become paler, the upperparts striped, the wing feathers grayer, and the belly acquires dark spots.
Haliotis laevigata is a species of marine
mollusc in the family
Haliotidae,
endemic to Tasmania and the southern and western coasts of Australia. This picture shows five views of a green H. laevigata shell, 7.5 centimetres (3.0 in) in length. The holes in the shell, characteristic of
abalones, are respiratory apertures for venting water from the gills and for releasing sperm and eggs into the
water column.
A
macro view of a Gonia capitatafly feeding on
honey, showing its
proboscis and
pedipalps (the two appendages protruding from the proboscis), two types of insect mouthparts. The proboscis actually comprises the labium, a quadrupedal structure, and a sponge-like labellum at the end. Flies eat solid food by secreting
saliva and dabbing it over the food item. As the saliva dissolves the food, the solution is then drawn up into the mouth as a liquid. The labellum's surface is covered by minute food channels which form a tube leading to the
esophagus, and food is drawn up the channels by
capillary action.
The maxima clam (Tridacna maxima) is a species of
bivalve found throughout the
Indo-Pacific. It is found on the surface of reefs or sand, or partly embedded in coral (as with this specimen), in the oceans surrounding east Africa, India, China, Australia, Southeast Asia and the islands of the Pacific. This clam is much sought after in the aquarium trade, as its often striking coloration—the result of crystalline pigment—mimics that of the true
giant clam.
A cross section of a post-
clitellum segment of an annelid (ringed worm); almost all segments of an annelid contain the same set of organs and parts, a pattern called
metamerism. Annelids have no lungs, but rather exchange
carbon dioxide and
oxygen directly through the moist skin when blood reaches the extremely fine capillaries of the body walls; a dry worm cannot breathe and will die of suffocation. The worm's red blood, which does not consist of
platelets or
red cells but mostly of a liquid containing suspended
hemoglobin, makes a circuit up and down the animal in its closed circulatory systems.
The paddyfield pipit (Anthus rufulus) is a
passerine bird in the family
Motacillidae, comprising
pipits,
longclaws and
wagtails. About 15 cm (6 in) in length and native to southern Asia, its plumage in both sexes is greyish-brown above and paler yellowish-brown below, with dark streaking on the breast. A bird of open country, pasture and cultivated fields, it sometimes makes short flights, but mostly runs on the ground, foraging for insects and other small invertebrates. The paddyfield pipit builds its cup-shaped nest in a concealed location on the ground, and may have two or more broods in a year. This A. r. rufulus individual was photographed in
Kanha Tiger Reserve, in the Indian state of
Madhya Pradesh.
Glaucus atlanticus is a species of small, blue
sea slug. This
pelagic aeolid
nudibranch floats upside down, using the surface tension of the water to stay up, and is carried along by the winds and ocean currents. The blue side of their body faces upwards, blending in with the blue of the water, while the grey side faces downwards, blending in with the silvery surface of the sea. G. atlanticus feeds on other pelagic creatures, including the
Portuguese man o' war.
Entries here consist of
Good and
Featured articles, which meet a core set of high editorial standards.
Mephitidae is a
family of
mammals in the
orderCarnivora, which comprises the
skunks and
stink badgers. A member of this family is called a mephitid. The skunks of the family are widespread across the Americas, while the stink badgers are in the
Greater Sunda Islands of southeast Asia. Species inhabit a variety of habitats, though typically grassland, forest, and shrubland. Most mephitids are 20–50 cm (8–20 in) long, plus a 10–40 cm (4–16 in) tail, though the
pygmy spotted skunk can be as small as 11 cm (4 in) plus a 7 cm (3 in) tail, and some
striped skunks can be up to 82 cm (32 in) plus a 40 cm (16 in) tail. No estimates have been made for overall population sizes of any of the species, but two species are classified as
vulnerable. Mephetids in general are not domesticated, though skunks are sometimes
kept as pets.
The twelve species of Mephitidae are split into four
genera: the
monotypicConepatus, hog-nosed skunks; Mephitis, skunks; Mydaus, stink badgers; and Spilogale, spotted skunks. Mephitidae was traditionally a
clade within the
Mustelidae family, with the stink badgers combined with other
badgers within the Melinae genus, but more recent genetic evidence resulted in the consensus to separate Mephitidae into its own family. Extinct species have also been placed into all of the extant genera besides Mydaus, as well as 9 extinct genera; 26 extinct Mephitidae species have been found, though due to ongoing research and discoveries the exact number and categorization is not fixed. (Full article...)
...that Caecillians, an order of
amphibians resembling snakes or worms, possess a pair of tentacles, located between their eyes and nostrils which are probably used for
olfaction?
Image 2A brilliantly-coloured
oriental sweetlips fish (Plectorhinchus vittatus) waits while two boldly-patterned
cleaner wrasse (Labroides dimidiatus) pick parasites from its skin. The spotted tail and fin pattern of the sweetlips signals sexual maturity; the behaviour and pattern of the
cleaner fish signal their availability for
cleaning service, rather than as prey (from Animal coloration)
Image 3Non-bilaterians include sponges (centre) and corals (background). (from Animal)
Image 31The red
pigment in a flamingo's plumage comes from its diet of shrimps, which get it from microscopic algae. (from Animal coloration)
Image 32A
praying mantis in
deimatic or threat pose displays conspicuous patches of colour to startle potential predators. This is not warning coloration as the insect is palatable. (from Animal coloration)
Image 33Simplified schematic of an island's fauna – all its animal species, highlighted in boxes (from Fauna)
Image 34Animal anatomical engraving from Handbuch der Anatomie der Tiere für Künstler. (from Zoology)
Image 37Idealised
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. (from Animal)
Image 38The bilaterian gut develops in two ways. In many
protostomes, the blastopore develops into the mouth, while in
deuterostomes it becomes the anus. (from Animal)
Image 44The microscopic cave snail Zospeum tholussum, found at depths of 743 to 1,392 m (2,438 to 4,567 ft) in the
Lukina Jama–Trojama cave system of
Croatia, is completely blind with a translucent shell (from Fauna)
The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species,[1] along with their principal habitats (terrestrial, fresh water,[2] and marine),[3] and free-living or parasitic ways of life.[4] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[5] Using patterns within the
taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[6][7][a]
^The application of
DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000
insect species for
Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (
Cecidomyiidae).[8]
^Stork, Nigel E. (January 2018). "How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth?". Annual Review of Entomology. 63 (1): 31–45.
doi:
10.1146/annurev-ento-020117-043348.
PMID28938083.
S2CID23755007. Stork notes that 1m insects have been named, making much larger predicted estimates.
^
abcdNicol, David (June 1969). "The Number of Living Species of Molluscs". Systematic Zoology. 18 (2): 251–254.
doi:
10.2307/2412618.
JSTOR2412618.
^Sluys, R. (1999). "Global diversity of land planarians (Platyhelminthes, Tricladida, Terricola): a new indicator-taxon in biodiversity and conservation studies". Biodiversity and Conservation. 8 (12): 1663–1681.
doi:
10.1023/A:1008994925673.
S2CID38784755.