1. Animals without backbones
Grade 4

2. invertebrates Invertebrates do not have a backbone.
Most animals on Earth do not have backbones!
Symmetry: when two parts of an object are identical along an imaginary dividing line.
For example, this shape can be divided along a midline:

3. Aquatic invertebrates
Aquatic animals live in the water
Many animals that live in water are invertebrates
Sponges:
the simplest invertebrates.
Made up of two cell layers.
Shaped like a sack with an opening.
Do not have symmetry
Cnidarians:
have tentacles, which are arm-like limbs with stinging cells on the end.
Some stay in one place while others swim.

4. sponges

5. Coral and sponges

6. Mollusks and echinoderms
All have shells
Most have shells outside the body and stay in one place
Some have shells inside the body and move
Echinoderms:
Have spiny skin
Have endoskeleton
Endoskeleton: internal support structure

7. Mollusks that move

8. Mollusks that stay in one place

9. echinoderms

10. arthropods Largest group of invertebrates They have:
Jointed legs
Body divided into sections
Exoskeleton
Hard covering that protects the body
Keeps moisture in
Examples are lobsters, bees, insects, spiders

11. Insects and centipedes
One pair of antennae
Three pairs of legs
One or two pairs of wings
Three body sections
Centipedes and millipedes
Long slender bodies and many legs
Centipede: one pair of legs on each section
Millipede: two pairs of legs on each section

12. Arachnids and crustaceans
Four to five pairs of legs
Two body sections
Fangs
Examples: spiders, ticks scorpions
Crustaceans:
Two pairs of antennae
Two or three body sections
Ability to chew
Most live in the oceans
Examples: shrimp, crabs, lobster

13. Some crustaceans

14. Centipedes and millipedes

15. Some insects

16. worms Three types: Flatworms Roundworms Segmented worms Flat
Three cell layers thick
Some live inside other animals
One opening for food and wastes
Roundworms
Round
One –way digestive system
Most live inside the bodies of other animals
Segmented worms
Bodies have segments, or sections
Separate opening to get rid of waste
Live on land; none live in another animal’s body