Invertebrates are most of the animal kingdom. Biologists sort them into groups by their body plans.
| Group | Body plan | Examples |
|---|---|---|
| Sponges | A body full of pores that filters food from water; stays in one place | Bath sponges |
| Cnidarians | Soft, jelly-like body with stinging tentacles | Jellyfish, corals, sea anemones |
| Worms | Long, soft body; earthworms have ring-shaped segments | Earthworms, leeches |
| Mollusks | Soft body, often a shell, a muscular foot | Snails, clams, octopuses |
| Arthropods | Jointed legs and a hard outer skeleton (exoskeleton) | Insects, spiders, crabs |
| Echinoderms | Spiny skin, body in five parts around a center | Sea stars, sea urchins |
Key idea
Arthropods are the biggest group. They include insects (6 legs, 3 body parts), arachnids (8 legs, 2 body parts: spiders, scorpions, ticks) and crustaceans (crabs, lobsters, shrimp).
Why it works
These groups are based on deep features of the body: how it is built, whether it has segments, what supports it. Where an animal lives does not decide its group. A jellyfish and a sea star live in the sea, but they have no backbone, gills or fins, so they are not fish.
Worked examples
Worked example: Name the group
An animal has jointed legs, a hard exoskeleton, eight legs and two body parts. Which group?
- Jointed legs and an exoskeleton: an arthropod.
- Eight legs and two body parts: an arachnid.
Answer: An arachnid (an arthropod).
Worked example: Spider or insect?
Is a spider an insect?
- Count legs: a spider has 8.
- Insects have 6.
Answer: No. A spider is an arachnid.
Common mistake
Calling jellyfish and starfish fish, and spiders insects. Jellyfish are cnidarians, sea stars are echinoderms, and spiders are arachnids, a different arthropod group from insects. Check: count the legs and look for a backbone, gills and fins.