VMBO Year 3 · Classification · Section 4.5

Arthropods and vertebrates

Features of arthropods

Arthropods have bodies organised into segments and jointed appendages. Their hard exoskeleton protects and supports them, but cannot expand continuously. To grow, they must moult, shedding the old covering and expanding before the new one hardens.

Four groups of arthropods

Body divisions and leg numbers help distinguish groups.

• Myriapods: centipedes and millipedes, with many trunk segments bearing legs.
• Crustaceans: animals such as crayfish, shrimp and woodlice. Familiar examples have ten or fourteen walking legs, but leg numbers vary across this large group.
• Arachnids: usually eight walking legs; spiders have a cephalothorax and abdomen. In harvestmen, these regions appear broadly joined.
• Insects: six legs attached to the thorax, and a body divided into head, thorax and abdomen. Examples include butterflies, beetles and flies.

Features of vertebrates

Vertebrates have a vertebral column, or backbone, within an internal skeleton. Their skin covering, temperature regulation, breathing organs and reproduction help identify the traditional school groups.

Five traditional vertebrate groups

• Fish: usually have scales and mucus, breathe using gills and are ectothermic. Many lay eggs without a shell in water, although exceptions occur.
• Amphibians: usually have moist skin. Many larvae breathe through gills; adults often use lungs and skin. They are ectothermic and typically lay jelly-covered eggs in water.
• Reptiles: have dry, scaly skin, breathe with lungs and are ectothermic. Many lay eggs with a leathery shell, while some give birth to live young.
• Birds: have feathers, breathe with lungs, produce much of their own body heat and lay hard-shelled eggs.
• Mammals: have hair at least during part of their lives, breathe with lungs and produce much of their own body heat. Most give birth to live young, and all feed their young milk; a few mammals lay eggs.

Habitats

Vertebrates occur in water, on land and in the air. Whales are marine mammals but must breathe air. Sea turtles live in the sea but lay eggs on land. Habitat alone therefore does not identify an animal's group.