BIOLOGY · Stage 7 · Chapter 1: Classification and Life
Biology 1.4 Classifying invertebrate animals
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Lesson summary
Biology 1.4 Classifying invertebrate animals — Alhussan International Academy — Biology (Stages 7–9) notes: Describe the main invertebrate groups and classify an animal using its features.Invertebrate = an animal without a backbone, and most multicellular species are animals, mos…
- Observation
- Small groups
- Features used
- Arthropods
- Crustaceans
- Echinoderms
Section 1 of 5What is an invertebrate
- Invertebrate = an animal without a backbone, and most multicellular species are animals, most of which are invertebrates.
- Observation
- plants and animals are multicellular, with millions of species, and observation and secondary sources such as books help to sort and classify them.
- Small groups
- most invertebrates are small and often unnoticed, and they are divided into smaller groups to help identification.
- Features used
- body covering, number of legs, whether the body is segmented and the presence of a shell.
- Size
- size is not a classification feature, since a tiny mite and a giant crab can both be arthropods.
Section 2 of 5Arthropods
- Arthropods
- all have a hard outer skeleton (exoskeleton) and jointed legs, in four groups.
- The exoskeleton protects the animal and limits water loss, but it cannot grow, so arthropods moult and grow a larger one.
- Insects
- three body segments, six legs, a pair of antennae on the head, and often wings.
- (head, thorax and abdomen)
- three body segments, six legs, a pair of antennae on the head, and often wings.
- Arachnids
- two body segments and eight legs, and many spin webs, including spiders, scorpions and ticks.
- Crustaceans
- at least 10 legs, such as crabs, prawns and shrimps, mostly living in water.
- Myriapods
- long segmented bodies with 1 or 2 pairs of legs on each segment, such as centipedes and millipedes.
| Group | Body sections | Legs | Other features |
|---|---|---|---|
| Insects | 3 | 6 | Antennae, often wings |
| Arachnids | 2 | 8 | No antennae, no wings |
| Crustaceans | 2 or more | 10 or more | Hard outer skeleton, mostly aquatic |
| Myriapods | Many segments | 1 or 2 pairs per segment | Centipedes and millipedes |
Section 3 of 5Other invertebrate groups
- Echinoderms
- spiny skins, star- or ball-shaped, such as sea urchins and starfish.
- Cnidarians
- all live in the sea, with no legs and stinging tentacles around the mouth, such as jellyfish, sea anemones and coral.
- Molluscs
- muscular bodies and no legs, though they may have tentacles, and many have a shell inside or outside the body.
- Slugs and snails live on land, while squids, octopuses and many shellfish live underwater.
- Worms
- no legs and long bodies, which may be flat, round or segmented.
- (many rings or sections)
- Some have hooks and suckers, such as flukes and tapeworms, and segmented worms (annelids) include earthworms.
- no legs and long bodies, which may be flat, round or segmented.
Section 4 of 5Placing an animal in a group
- Backbone
- if there is none, it is an invertebrate.
- Jointed legs and a hard outer skeleton
- it is an arthropod.
- Count legs and body sections
- this gives the class; a garden spider has eight legs and two sections, so it is an arachnid and not an insect.
- A woodlouse has seven pairs of legs and lives in damp places, so it is a crustacean.
- An earthworm has no legs and a body of rings, so it is an annelid.
- Evidence
- base the decision on observable body features and not on habitat or appearance.
Section 5 of 5Importance of invertebrates
- Ecosystems
- invertebrates pollinate plants, break down dead material and form the base of many food chains.
- Fieldwork
- animals can be collected with a pooter or pitfall trap and sorted using a short key.
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