annelidA worm whose body is a chain of ring-like segments. Annelid goes back to anellus, a Latin word meaning little ring. Ragworms, earthworms and leeches are annelids, over 22,000 species.
arachnidAn eight-legged arthropod with no antennae. Spiders, scorpions, ticks and mites are arachnids. The Greek word arachne means spider.
arthropodA member of the phylum of insects, spiders and crabs. Its armor is made of chitin, and its legs come in jointed pairs. There are more species of arthropods than of all other phyla put together.
cnidarianA jellyfish, coral or other bearer of stinging cells. Sea anemones and hydras are cnidarians too, over 11,000 species in all. The gut has one opening, often ringed by tentacles.
crustaceanA crab or other arthropod with four antennae. The name is Latin for those with shells. Shrimp, lobsters, barnacles and krill are crustaceans, about 67,000 species.
echinodermA sea-floor animal built on a five-part plan. Sea stars, sea urchins and sea cucumbers are echinoderms, about 7,600 species. None lives in fresh water or on land.
Show all 36 terms (30 more)
Groups of invertebrates, continued
flatwormA worm so thin that oxygen soaks straight in. Its phylum is Platyhelminthes. The gut has a single opening, which takes food in and lets waste out. Planarians and tapeworms are flatworms.
insectA six-legged arthropod, often winged. Its body has three parts: head, thorax and abdomen. It breathes through paired openings along its sides.
invertebrateAny animal that never grows a spinal column. The word says what the animal lacks, not what it is: sponges, worms, insects and squid are grouped together only because none has a backbone.
molluskAn animal with a mantle, the organ that makes shells. Snails, clams, squid and octopuses are mollusks, about 86,600 living species. Gastropods, bivalves and cephalopods are three of the classes. Only the arthropods are a bigger phylum.
roundwormA nematode: a worm sheathed in a flexible cuticle. Its gut is a tube with an opening at each end. Most species live free; many others are parasites.
spongeAn animal whose whole body works as a water strainer. The phylum is Porifera. About 150 of its species live in fresh water and the rest in the sea. Its cells do not form tissues.
Body plans and parts 12
antennaA jointed feeler that smells, tastes and touches. The plural is antennae. Insects have one pair and crustaceans two; arachnids have none.
appendageA part that sticks out from the body, as a leg does. Legs, antennae and mouthparts are appendages of an arthropod, and they come in pairs.
bilateral symmetryA body whose two sides are mirror images. The one plane that divides it runs from head to tail. Worms, insects and people have it; echinoderms have it only as larvae.
cephalothoraxThe fused head and chest of a spider or a crab. The word joins the Greek for head and thorax. An arachnid's eight legs are attached to it. An insect's head stays separate.
compound eyeAn eye made of many small lenses side by side. Insects and crustaceans have them. The picture is coarse, but the eye takes in a very wide angle and catches fast movement.
exoskeletonArmor that doubles as an animal's skeleton. It holds the body's shape from outside and shields the organs. A snail's shell is one, and so is a crab's.
hydrostatic skeletonA body held firm by fluid under pressure. Soft-bodied animals such as worms have one. Muscles in the body wall squeeze the fluid to make the body longer or shorter.
nematocystThe tiny capsule that fires a jellyfish's sting. It holds a coiled, barbed thread. The cell around it, a cnidocyte, is used once and then replaced.
radial symmetryA body arranged like the spokes of a wheel. A jellyfish has it, and so does an adult starfish, with its matching arms.
spiculeOne of the hard needles that stiffen a sponge. They are made of silica or of calcium carbonate. Meshed together they are the sponge's skeleton.
spinneretA spider's silk outlet, studded with tiny spigots. Most spiders have six. Each spigot makes one filament. Some insect larvae have spinnerets too.
water vascular systemThe network of water-filled canals in a starfish. It works the tube feet: muscles force water into a foot to stretch it out. Echinoderms use it to move, to feed and to breathe.
Growth and ways of life 12
buddingMaking offspring from an outgrowth of the parent. A hydra buds: a small copy grows on its side, then breaks away. The young animal is a clone of its parent.
cephalizationThe gathering of nerves and senses at the head end. It goes with bilateral symmetry and with moving forward. It has led to brains in arthropods, in cephalopods and in vertebrates.
hostAn organism that shelters and feeds a parasite. Many parasitic worms live in hosts. Many tapeworms need more than one kind of host in a lifetime.
medusaThe free cnidarian form, shaped like an umbrella. A jellyfish is a medusa. Its tentacles trail from the bell, and its larvae settle and grow as polyps.
metamorphosisThe big change that turns a grub into a beetle. Complete metamorphosis has four stages: egg, larva, pupa and adult. Incomplete metamorphosis has three, egg, nymph and adult.
moltingClimbing out of an old exoskeleton to grow. Ecdysis is the scientific name. The new covering is pale and soft at first and hardens within an hour or two.
nerve netA brainless nervous system spread through the body. Cnidarians and echinoderms have nerve nets. They go with radial symmetry.
open circulatory systemA system where body fluid washes freely over organs. Arthropods have one. The fluid, called hemolymph, does the work of blood and flows through a body cavity.
polypThe attached, tube-shaped form of a cnidarian. A hydra is a polyp. One end is fixed to a surface, and the mouth, ringed by tentacles, is at the other.
regenerationRegrowth of a lost arm, tail or other body part. Planarians are best known for it. Most starfish can regrow a lost arm.
sessileStaying attached to one spot, as sponges do. Corals are sessile too. A sessile animal moves only if something outside it, such as a current, moves it.
zooplanktonAnimal drifters of open water, mostly tiny. The name is Greek for animal drifter. They cannot swim against a current, so they go where the water goes.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Invertebrate, Cnidaria, Flatworm, Nematode, Annelid, Mollusca, Arthropod, Echinoderm, Insect, Arachnid, Crustacean, Exoskeleton, Skeleton, Hydrostatic skeleton, Compound eye, Antenna (zoology), Spinneret, Cnidocyte, Water vascular system, Tube feet, Cephalothorax, Appendage, Sponge spicule, Moulting, Ecdysis, Metamorphosis, Holometabolism, Hemimetabolism, Polyp (zoology), Jellyfish, Sessility (motility), Budding, Regeneration (biology), Planarian, Starfish, Nerve net, Cephalization, Host (biology), Cestoda, Zooplankton and Colossal squid. Radial symmetry, bilateral symmetry, nematocyst, polyp, medusa, sessile, cephalothorax and nerve net rest on their entries in Wiktionary, the free dictionary (https://en.wiktionary.org/). The sponge, the cnidarian's single opening, the roundworm's cuticle, the segments of annelids, the antennae of crustaceans and the count of arthropod species were checked a second time against the phylum and class pages of the Animal Diversity Web of the University of Michigan Museum of Zoology (https://animaldiversity.org/accounts/Porifera/, and the same address for Cnidaria, Nematoda, Annelida, Arthropoda and Crustacea). The fact after each meaning is from the same pages. Licence: own work
How it works
This invertebrate vocabulary worksheet prints the animal groups of the unit, sponge, cnidarian, flatworm, roundworm, annelid, mollusk, arthropod and echinoderm among them, to match to their meanings. The answer key prints last.
Read the full guide
Body plans and parts
Set opens on Groups of invertebrates. The second set has exoskeleton, radial symmetry, bilateral symmetry, hydrostatic skeleton, compound eye, antenna, spinneret, nematocyst, water vascular system, cephalothorax, appendage and spicule.
Growth and ways of life
The third set has molting, metamorphosis, polyp, medusa, sessile, budding, regeneration, open circulatory system, nerve net, cephalization, host and zooplankton.
Gastropod, bivalve and cephalopod, and larva, pupa and nymph, are named in the facts under the tool, not on the sheet.
Questions and answers
What percentage of animals are invertebrates?
By one estimate, 97 percent of all animal species. Insects alone account for more than half of the animal species described so far, with over a million named.
What is the largest invertebrate?
The colossal squid, going by weight: one has been confirmed at 495 kilograms (about 1,090 pounds). At the other end, the smallest invertebrates are about a hundredth of a millimeter long.
Do invertebrates have skeletons?
Many do, though never one of bone. Arthropods and most mollusks wear a hard exoskeleton, soft worms hold their shape with fluid under pressure, and echinoderms carry small hard plates in the skin.
Is this an invertebrate vocabulary worksheet for high school biology?
It works for one: the second and third sets hold the longer terms of a biology course, such as hydrostatic skeleton, nematocyst, water vascular system, cephalization and open circulatory system. The sheet prints no grade.