Invertebrates
0Loading crowd target
0Loading crowd target
7correct to beat the crowd
50 free Invertebrates trivia questions with answers — trivia quiz, new questions added Oct 2026.
Animals without a backbone make up the overwhelming majority of animal species, yet most of us could name only a few of their groups. Among them are the cleverest hunters in the sea, the builders of the largest living structure on Earth and creatures that can survive a trip into space. This quiz covers what an introductory biology or zoology course expects: the main invertebrate phyla and classes and how to tell them apart, body plans and symmetry, skeletons, breathing and circulation, life cycles, and a few insect and crustacean orders. Mixed in are the record holders, the oddities and the naturalists who first sorted these animals into groups. It suits students revising invertebrate classification, teachers who want a ready-made multiple-choice round, and anyone curious about the spineless side of the animal kingdom. Every answer comes with a short explanation.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Which soft-bodied invertebrate has three hearts, bluish copper-based blood and two-thirds of its neurons in its arms?
Octopus
Octopus blood is blue because it carries oxygen with hemocyanin rather than iron-rich hemoglobin. One main heart serves the body while two smaller ones push blood through the gills.
Q 02Which Antarctic deep-sea animal, weighing about 1,000 pounds, is the heaviest living invertebrate known to science?
Colossal squid
The colossal squid also has the largest eyes documented in the animal kingdom, 27 to 30 cm across. The heaviest confirmed specimen weighed 495 kg.
Q 03Which microscopic eight-legged invertebrates can dry into a dormant state and survive the vacuum of outer space?
Tardigrades
Tardigrades, nicknamed water bears, pull in their legs and shrivel into a lifeless-looking form called a tun. In that state they have endured vacuum, radiation and cold close to absolute zero.
Q 04Which armored sea creature has bright blue blood that drug makers use to test injectable medicines for bacterial toxins?
Horseshoe crab
The horseshoe crab is no true crab but a closer relative of spiders. Cells in its copper-based blood clot around bacterial toxins, which makes it a sensitive safety test.
Q 05Which spiny-skinned sea animal can regrow lost arms and feeds by pushing its stomach outside its body onto prey?
Starfish
Starfish pry a clam's shell open a crack, then slip their stomach inside to digest the soft tissue. In some species a single severed arm can regrow a whole new animal.
Q 06Which drifting sea animals have no true brain, yet one species can revert to a juvenile stage after maturing?
Jellyfish
Jellyfish coordinate their swimming with a ring of nerves instead of a brain. The so-called immortal species, Turritopsis dohrnii, can turn back into a polyp after it has matured.
Q 07Which insects form Coleoptera, the largest order of animals, making up about a quarter of all known animal species?
Beetles
Beetles number about 400,000 described species, more than any other order. Their front wings are hardened into cases called elytra that shield the flight wings underneath.
Q 08Which tiny colonial animals, each a polyp secreting limestone, built Australia's Great Barrier Reef?
Corals
Corals built the world's biggest single structure made by living organisms, one visible from space. Each polyp is typically only a few millimeters across.
Q 09Which small sea creature is a vertebrate rather than an invertebrate, with males that carry the developing young?
Seahorse
The seahorse is a bony fish with a backbone, a relative of the pipefishes. In place of scales it has thin skin stretched over rings of bony plates.
Q 10Which animals of the phylum Porifera have no nervous or digestive system and feed by filtering water through pores?
Sponges
Sponges may be the sister group to all other animals, though the evidence is not settled. Most spend adult life fixed to the seabed, pumping water through their bodies for food and oxygen.
Q 11Which phylum of insects, spiders and crustaceans accounts for over 80 percent of all known living animal species?
Arthropoda
Arthropoda takes its name from the Greek for jointed leg. Every member has a segmented body, an external skeleton and paired limbs that bend at joints.
Q 12Snails, clams and cuttlefish all belong to which soft-bodied phylum, the animal kingdom's second-largest by species?
Mollusca
Mollusca accounts for about 23% of all named marine organisms, more than any other phylum in the sea. Most members have a muscular foot and a mantle whose edge secretes the shell.
Q 13Sea urchins, sand dollars and brittle stars belong to which phylum, the largest found only in the sea?
Echinodermata
Q 21Sea anemones and hydras capture prey with which specialized stinging cells that fire a toxin-filled capsule on contact?
Cnidocytes
Cnidocytes are single-use cells: once the hair-like trigger is touched and the capsule fires, the whole cell has to be replaced.
Q 22Sea anemones and adult sea urchins usually have parts repeating around a central axis, showing which type of symmetry?
Radial
Radial symmetry suits anchored, floating and slow-moving animals, which meet the world from every side. A two-sided body instead favors streamlined, directed movement.
Q 23Sea anemones and many soft-bodied worms hold their shape with fluid under pressure, forming which kind of skeleton?
Hydrostatic
Echinodermata means hedgehog skin in Greek. Its members start life as two-sided larvae but grow into adults built on a five-part plan, and they live on the sea bed at every depth.
Q 14Sea squirts and lancelets have no backbone, yet they belong to which phylum alongside fish, birds and humans?
Chordata
Chordata membership rests on the notochord, a stiffening rod, rather than on a backbone. Sea squirt larvae have one, along with a hollow nerve cord, while they swim freely.
Q 15Spiders, mites and harvestmen belong to which class of eight-legged invertebrates that have no antennae or wings?
Arachnids
Arachnids take their name from arachne, the Greek word for spider, shared by the boastful weaver of myth who was turned into one. Adults walk on eight legs, where adult insects have six.
Q 16Snails, slugs and whelks belong to which class of animals, at home in the sea, fresh water and on land?
Gastropods
Gastropods are named from the Greek for stomach and foot, because the crawling foot sits beneath the gut. They have spread through the sea, fresh water and dry land.
Q 17An adult insect's body, which carries six legs and usually wings, is divided into which sections?
Head, thorax and abdomen
Head, thorax and abdomen each have a job: the first carries the antennae and jaws, the second bears every leg and wing, and the third holds most of the gut.
Q 18Which tough material, also found in the cell walls of fungi, forms the exoskeletons of insects and crustaceans?
Chitin
Chitin is the second most abundant polysaccharide in nature, behind only cellulose. Many crustaceans stiffen theirs further with calcium carbonate.
Q 19Because a rigid exoskeleton cannot stretch, which process must insects, spiders and lobsters repeat in order to grow?
Molting
Molting means shedding the old outer covering to reveal a new one grown beneath it. Until that fresh covering hardens, the animal is pale, soft and easy prey.
Q 20Which soil-dwelling invertebrate breathes through its skin and was the subject of Charles Darwin's last scientific book?
Earthworm
The earthworm takes in oxygen straight through its moist skin. Darwin argued that few other animals have played so important a part in the history of the world.
Hydrostatic skeletons work because the trapped fluid cannot be squeezed smaller. When muscles tighten around one part of the body, the pressure forces another part to lengthen.
Q 24Which parasitic flatworms have no gut, absorbing nutrients through their body surface inside a host's intestine?
Tapeworms
Tapeworms anchor themselves to the intestine wall with a scolex, a knob armed with hooks, suckers or both. Behind it, the neck keeps budding off new body segments.
Q 25Caenorhabditis elegans, the first animal to have its whole genome sequenced, is which kind of transparent invertebrate?
Roundworm
The roundworm C. elegans, a nematode about 1 mm long, is so thoroughly studied that biologists have mapped the developmental fate of every single cell in its body.
Q 26Which flatworms, favorites of regeneration research, can split into fragments that each regrow a complete animal?
Planarians
Planarians owe the feat to abundant adult stem cells called neoblasts, which renew themselves and can produce every cell type the body needs.
Q 27Which blood-feeding segmented worm returned to hospitals in the 1980s to relieve blood congestion after microsurgery?
Leech
The leech secretes hirudin, a peptide that stops blood from clotting while it feeds. Doctors now also use hirudin itself as an anticoagulant drug.
Q 28Snails and slugs scrape up food with which minutely toothed ribbon, often compared to a tongue?
Radula
The radula carries many rows of tiny teeth, and fresh rows move up from the rear as the front ones wear out. Some predatory slugs use theirs to seize and devour live prey.
Q 29Pearls are built from layers of which iridescent material, which also lines the inside of an oyster's shell?
Nacre
Nacre, also known as mother-of-pearl, is made of microscopic tablets of aragonite, a form of calcium carbonate, stacked in sheets that make it strong and resilient.
Q 30Which shelled cephalopod, often called a living fossil, grows by adding ever-larger chambers to its coiled shell?
Nautilus
The nautilus moves into each new, larger chamber and seals the old one behind a wall. Its line has survived relatively unchanged for hundreds of millions of years.