Arthropods: a jumping spider, Phidippus audax. Arthropods, which include insects, crustaceans, mites, spiders and crabs, are the largest animal lineage, accounting for 80% of all known animal speciesPhotograph: Animal Earth/Thames & HudsonArthropods: Compound eyes comprising thousands of hexagonal units are a characteristic of many arthropods, including this parasitoid wasp Photograph: Tomas Rak/Animal Earth/Thames & HudsonArthropods: a cynipid wasp. Some insects have simple eyes in addition to their compound eyes, three of which can be seen on top of this wasp's headPhotograph: Tomas Rak/Animal Earth/Thames & HudsonCnidarians: These aquatic animals come in a bewildering variety of forms and include jellyfish, sea anemones, corals, sea pens, sea fans, sea daisies, hyrdoids and hydras. What looks like a single individual is often a colony of polyps with specialised functions. In Porpita (example shown here) there are polyps for providing buoyancy, feeding (in the form of tentacles), digestion and reproductionPhotograph: Arthur Anker/Animal Earth/Thames & HudsonCnidarians: This one's a jellyfish (Bougainvillia superciliaris) with a hitchhiking amphipod (Hyperia galba)Photograph: Alexander Semenov/Animal Earth/Thames & HudsonTunicates: These are exclusively marine. Some have a sponge-like form, living their entire adult lives glued to the spot, while others, like this one (Pegea confoederata), form floating colonies Photograph: Kevin Lee/Animal Earth/Thames & HudsonHemichordates: An acorn worm (Ptychodera flava) in its hat-shaped, planktonic stage. The hemichordates are rarely seen and poorly known. They comprise burrowing acorn worms and free-floating, tubular coloniesPhotograph: Eric Roettinger/Animal Earth/Thames & HudsonEchinoderms: a sea urchin, Coelopleurus floridanus. The mobile spines offer protection from predators. Since this species lives in relatively deep water, the purpose of its bright pigments is unknown. As well as star-shaped forms, the echinoderms include sedentary species that look a bit like plants, and species that burrow in the sedimentPhotograph: Arthur Anker/Animal Earth/Thames & HudsonBrush-heads (Loricifera): Pliciloricus sp. These fantastical creatures were only discovered in 1974 and not described formally by science until 1983. Only a fraction of a millimetre long, the complex anterior structure is called an 'introvert' and sprouts a telescopic mouth cone, several stylets and hundreds of spines. The trunk is encased in plates like a suit of armour Photograph: Phil Miller/Animal Earth/Thames & HudsonAnnelids: Bristle worms, beard worms, spoon worms, peanut worms, earthworms and leeches. Segmentation – clearly visible here – is a distinguishing feature of all the annelidsPhotograph: Alexander Semenov/Animal Earth/Thames & HudsonAnnelids: a tube-dwelling polychaete (meaning 'many bristles'). Many annelids have elaborate, colourful tentacles for filter-feeding and breathing. The funnel-shaped structure seals the tube when the animal retreats insidePhotograph: Alexander Semenov/Animal Earth/Thames & HudsonMolluscs include slugs and snails, octopuses and squids, chitons, bivalves, scaphopods, solenogasters and caudofoveates. This marine mollusc (Coryphella polaris) is a nudibranch, commonly known as sea slugsPhotograph: Alexander Semenov/Animal Earth/Thames & HudsonMolluscs (Chromodaris annulata): the bold colours and patterns of the sea slugs warn predators of their toxicityPhotograph: Arthur Anker/Animal Earth/Thames & HudsonMolluscs: a sea angel (Clione limacia)Photograph: Alexander Semenov/Animal Earth/Thames & Hudson