Year 1 · Classification · Section 3.3

Animals

Symmetry

Symmetry describes how an organism's body can be divided into matching halves. There are three patterns:

• Bilateral symmetry: One main plane divides the body into mirror-image halves, as in a beetle or a human.
• Radial symmetry: Several planes through the centre divide the body into matching halves, as in many jellyfish.
• Asymmetry: No plane divides the body into matching halves, as in many sponges.

The human body is described as bilaterally symmetrical, although its internal organs are not arranged symmetrically.

Skeletons

A skeleton consists of structures that support and protect the body. This course distinguishes:

• Internal skeleton (endoskeleton): Located inside the body, as in humans and many sponges.
• External skeleton (exoskeleton): Located outside the soft tissues, as in insects and shelled snails.
• No hard skeleton: Animals such as jellyfish and many sea slugs lack a hard skeleton and often live in water.

Classifying animals

Symmetry and skeleton type help distinguish major animal groups. The animal kingdom contains many phyla; the exact number recognised depends on the classification system. Six groups studied here are:

• Sponges: Usually asymmetrical, with supporting fibres or mineral structures between their cells. Adults live attached to a surface, often on the seabed.
• Cnidarians: Usually radially symmetrical aquatic animals that catch prey using tentacles. Many, such as jellyfish, have no hard skeleton.
• Molluscs: Usually have a bilateral body plan and often a shell providing support or protection.
• Echinoderms: Adults usually show radial symmetry. They have an internal skeleton of calcareous plates and often spiny or bumpy skin. They live in the sea.
• Arthropods: Bilaterally symmetrical animals with an exoskeleton. Groups include arachnids such as spiders, insects such as flies, crustaceans such as crabs, and centipedes and millipedes.
• Vertebrates: Bilaterally symmetrical animals with an internal skeleton and backbone. Groups include bony fish, amphibians, reptiles, birds and mammals. Examples include surgeonfish, tree frogs, sand lizards and badgers.

Vertebrates are a subgroup within the phylum Chordata rather than a separate phylum in modern classification.