Every animal on Earth belongs to one of about 35 groups called "phyla". Some are familiar, but others are profoundly strange.
The number of species per phylum is always changing, as more animals are discovered. In 2014 alone, over 1,400 new marine species were discovered, and the largely unexplored deep sea surely holds many more surprises. Also, seemingly separate phyla sometimes turn out to be the same.
As a result, this list of phyla is not definitive. But it is our best attempt to sum up the vast diversity of Earth's animal life.
Acanthocephala, or spiny headed worms, are parasitic creatures. Adults live in the digestive tracts of vertebrates like fish. They are usually about 20cm long but some are known to be larger than 60cm.
Their spines are not just for show. The worms use them to literally anchor themselves onto their hosts. Some species also have a hook for even firmer attachment.
The worms pictured are acoel flatworms. Like all others in the Acoelomorpha phylum they do not have a gut or anus. They are remarkably resistant and can reportedly regrow from cells after being pressed through a sieve. It's not clear whether Acoelomorpha are distinct enough to be a true phylum.
This beautiful dragon-shaped ragworm belongs to the annelid phylum. Each of the segments you can see has small appendages, which help it move along. They also act like gills, usually in marine environments, though also sometimes freshwater and terrestrial.
The worm in the photo is one of 17,590 annelid species: others include earthworms, lugworms and leeches. Annelids have long been thought to be a sister group of arthropods, as they both have segmented bodies, says Philipp Schiffer, an evolutionary biologist at the University of Cologne in Germany. They are now grouped with molluscs.
The image shows a garden spider feeding on a common wasp caught in its web. These animals are both arthropods.
The arthropod phylum includes all insects, spiders and millipedes, as well as crustaceans and sea spiders. The long-extinct trilobites were also arthropods. Arthropoda is currently the largest phylum, with well over a million species.
Brachiopoda, also known as lamp shells, live in the ocean in very cold water. They were once much more diverse than they are today: there are many thousands of fossil species, but less than 450 living ones.
They look a bit like shelled molluscs like oysters or mussels. But whereas molluscs have their shells on their left and right sides, brachiopods have theirs on their upper and lower surfaces. They are closely related to bryozoans and phoronids.
Bryozoa are also known as sea mats, and look a bit like corals. They live on the sea floor, mostly on stones or algae, although a few are free-living. The tiny individuals in the colonies use their fine tentacles to filter particles of food from the water.
New individuals, called zooids, are budded from the body wall of an existing zooid or from a larva. All species have hermaphroditic colonies, but several also have male and female zooids, says Claus Nielsen of the Natural History Museum of Denmark in Copenhagen.
Some hermit crabs use bryozoans as a disguise: they let the bryozoans grow on their snail-shell house forming sturdy outgrowths, which gives some protection from predators. There are almost 6,000 living species and many more only known from fossils.
Chaetognaths are marine predators. They have an impressive set of grasping spines and jaws with which to capture their victims, which they then immobilise with neurotoxins. They are called "arrow worms" because of the hooked spines on each side of their heads.
Arrow worms are among the main constituents of plankton, making them very important in marine food webs. They are found throughout the ocean but there are only just over 100 species. It is not clear how they relate to each other.
All the animals pictured here are chordates. It is probably the most well-known phylum. It includes all animals with backbones, or "vertebrates", which includes birds, reptiles, fishes, amphibians and mammals.
There are over 64,000 living species, and many more known fossil chordates. The largest chordates are also the largest animals known to have existed: blue whales.
This lion's mane is the largest known jellyfish and belongs to the Cnidaria phylum. Most are marine predators, although some also live in freshwater. Their stinging tentacles, which shoot out specialized harpoon-like cells called cnidocytes, help them ensnare prey.
As well as jellyfish, this phylum also includes sea anemones.
This comb jelly is part of the Ctenophora phylum. They are also known as sea gooseberries, sea walnuts and Venus's girdles.
Whatever you call them, comb jellies are jelly-like marine predators. They eat jellyfish, zooplankton, larvae and sometimes even other ctenophores. Some are just a few millimetres long but the largest can be up to 2m.
This photo of a symbion is from Kaldbak Fjord in the Faroe Islands. It was taken in 1990, five years before symbions, and the Cycliophora phylum to which they belong, were described.
These strange creatures were discovered living in their thousands on the mouths of Norway lobsters. There are currently only two known species, so they remain little understood.
These starfish are echinoderms. These are found in 100m-deep water and feed on mussels and snails, among other smaller species.
The echinoderm phylum includes over 20,000 living species, including sea urchins, sea cucumbers and sea lilies.
There are roughly 169 species of goblet worm. Their bodies are shaped like goblets or cups, and are usually small and transparent. They contain a gut and reproductive organs, and around the upper rim they have a horseshoe-shaped line of tentacles for capturing particles of food.
The species pictured here is called Loxosomella. Goblet worms usually live in groups on stones and algae, but solitary species like Loxosomella are usually associated with other organisms, such as worms or sponges.
These are tiny water-dwelling worms. They are sometimes called "hairybacks", perhaps because they use hair-like organelles called ciliates to propel themselves forwards. They also have similar hairs near their heads, which they use to gather food - such as algae - into their mouths. They are rarely longer than 1mm.
Gnathostomulida is a phylum of microscopic worms discovered in the 1950s. At first they were thought to be flatworms (Platyhelminthes, phylum 28) but it now seems they are a separate phylum.
The worms mostly look rather simple, but they do have an impressive set of jaws, which is why they are also called jaw worms. There are just over 100 species.
Hemichordata are soft-bodied and live close to the ocean floor. They are worm-like, and many of them are called "acorn worms".
They and are among the most ancient phyla, dating back to the Cambrian period over 500 million years ago. They are closely related to echinoderms.
Kinorhynchs are found everywhere where there is mud or sand, but usually occur in relatively low numbers. Known as "mud dragons", they are another ancient group, with roots that go back to the Cambrian.
We know very little about them. Though they were discovered in the mid-1800s, fewer than 10 people have worked on them. Today there are about 220 known species, says Martin Vinther Sørensen of the Natural History Museum of Denmark in Copenhagen.
It would not be surprising if you have never seen one of these before. Loricifera are microscopic and were only discovered in 1983. There are 30 known species and they live in marine sediments.
They have a very complex life cycle: each animal has to go through several larval stages, such as "postlarvae" and "ghost larvae". They can also switch between sexual and asexual reproduction.
Despite their tiny size, an adult's mouth has up to nine appendages called scalids, which you can see in the image above. Some have up to 200 of these on their bodies, which help them move about.
Micrognathozoa are some of the smallest animals known to exist, clocking in at 125 micrometres. They also represent the most recently discovered animal phylum, having been formally described in 2000. There is only one known species: Limnognathia maerski, which was discovered in Greenland.
Despite their size, micrognathozoans have a highly complex set of jaws containing more than 16 tiny, movable elements. It remains a riddle how they can survive the eight-month Greenland winter, says Sørensen.
This nudibranch, or "sea slug", is a mollusc. This diverse phylum also includes clams, snails, slugs, squid and octopuses. It is the largest marine phylum.
One of the most elusive molluscs is the giant squid. Despite their fearsome reputation they are thought to be quite passive predators, whereas the smaller Humboldt squid is much more violent.
The picture shows a hatching dog roundworm, a nematode found in dog faeces. Like this species, more than half of all nematodes are parasites of plants and animals.
Though most are small, this phylum includes the genus Ascaris, which can live in human intestines and grow over 30cm long. Some parasitic species inflict or transmit severe human diseases, while others are among the most devastating crop pests. Many feed on bacteria, algae and other microorganisms, including other nematodes, and help to recycle nutrients in soils and sediments.
There is a saying, Schiffer says, that if you were to take away all the hills and mountains, the Earth's contour would still be visible, because of the nematodes. They have conquered almost every habitat, including Antarctica, deep gold mines in Africa and even the placentas of sperm whales where they can grow to several metres.
Horsehair worms are mostly aquatic and often live in streams or puddles. They are parasites of arthropods, including insects such as crickets. One female can lay several thousand eggs in one go. They often tie themselves into knots and so are sometimes called gordiids, after the legendary Gordian knot.
Thin and long, ribbon worms have no segments to separate their heads from their bodies. They look a bit like flatworms (phylum 28), but are clearly distinct: they have a mouth and anus.
They are sometimes called proboscis worms. Each worm has a tube-shaped organ called a proboscis hidden inside its body. This can pop out and grasp prey. Some species even have a sharp tip at the end of it.
Velvet worms are relatives of arthropods, the group that includes insects, although they also resemble annelid worms. They live in tropical and subtropical areas.
Velvet worms have a cunning way of getting food. They can shoot out a sticky, slimy fluid that effectively immobilises their prey. They can shoot the fluid up to 30cm.
These are microscopic parasites of marine species like flatworms and molluscs. They are extremely simple organisms, and we do not understand much about them. About 20 species are known but there could be up to 100, says George Slyusarev of Saint Petersburg State University in Russia.
These odd creatures are also known as horseshoe worms. Generally found in shallow waters, they are filter feeders, meaning they use a specialised organ to strain water for edible nutrients. There are only 10 species in this phylum. Some of them can use special tubes to attach or bore into rocks. They tend to be hermaphrodites.
Placozoans - the white bits of the above image - are truly enigmatic. So far the phylum only includes one species, Trichoplax adhaerens.
They have the simplest structure of any free-living animal, with only a handful of cell types, says Neil Blackstone of Northern Illinois University in DeKalb. Key aspects of their life cycle, including how they reproduce, have never been fully described.
The photograph shows a flatworm crawling over a coral reef. There are almost 30,000 species of flatworms, all of them soft-bodied. Many are parasitic, including the famous tapeworms.
Many flatworms are hermaphrodites with intricate reproductive organs. Their reproductive cycles can be very complex, with alternating sexual and asexual generations, says Schiffer.
Some flatworms, known as planarians, have staggering abilities that make them "potentially immortal", says Schiffer. "They can be cut into many small pieces, each of which will regenerate into a complete worm."
Sponges are filter feeders that suck up plankton and other floating organic matter. Many litres water can filter through them, so they can even survive in "ocean deserts" where the water contains few nutrients. Like flatworms, they do not have a digestive tract or an anus.
One look at the photograph should clarify why these animals are also known as "penis worms". Their formal name comes from a Greek god of fertility, Priapus. There are less than 20 known species and they live on the sea bed from the tropics to the polar regions. Their size varies widely, from 1mm to 40cm.
31. Rhombozoa, or Dicyemida
These thin worms are usually less than 1.5mm long. Rhombozoans are parasites that live in the appendages of cephalopods such as squid and octopuses. They are closely related to roundworms, and not all scientists agree that they belong to a completely separate phylum.
There are more than 1,500 rotifers, also known as wheel animals. The one pictured is a freshwater bdelloid rotifer, a group that is famous for having almost entirely given up sex. At the bottom left you can see the crown of tentacle-like cilia and the mastax, which is used for crunching food.
Also called peanut worms, this phylum has an estimated 144 species. There is some debate about whether or not peanut worms should have their own phylum, as they could simply be rather strange annelids.
Tardigrades, also known as water bears, are extremely robust. These 1mm-long creatures have been brought back to life after spending 10 days in space and they can cope with radiation levels 1000 times greater than humans can. They tend to live in damp places like lichen and mosses, or in tiny ponds on glaciers. They can survive at temperatures colder than have ever been observed on Earth.
Though they were discovered way back in 1915, there are still only two known species of these small, yellow-brown worms. They are about 1.5cm in size and have no brain or gut. Scientists have long debated whether these aptly-named "strange worms" should belong to their own phylum, as genetically they are very similar to Acoelomorpha.