Zoology · Ch 2 — Kingdom Animalia
Patterns of Symmetry
Patterns of Symmetry
Symmetry describes how an animal's body parts are arranged relative to a central axis, and it is closely tied to how an animal lives: sedentary or drifting animals tend toward radial symmetry, while actively moving, predator/prey-driven animals evolve bilateral symmetry.
An asymmetrical body has no consistent plan at all — no plane through the centre divides it into equal halves. This is the simplest possible body plan, seen in sponges and in adult gastropod snails.
A radially symmetrical body can be divided into two identical halves by any plane passing through its central axis, giving it a top and bottom but no left/right or front/back — it is the principal symmetry of diploblastic Radiata (Cnidaria). A special case, pentamerous radial symmetry, is seen in triploblastic echinoderms (e.g. starfish), whose adult body is organised into five repeating segments around a central axis, even though their larvae are bilaterally symmetrical.
Biradial symmetry, unique to ctenophores, is a hybrid: the body has only two planes of symmetry (through the longitudinal-sagittal axis, and through the longitudinal-transverse axis) rather than the many planes of true radial symmetry. …
What this figure shows. A simple irregular, lumpy sponge body outline demonstrating that no plane through its centre divides it into two equal halves — the simplest and least-organised body plan in the animal kingdom, also seen in adult gastropod snails. …
What this figure shows. Two examples side by side: a sea anemone shown from above with tentacles arranged evenly in a circle around a central mouth (true radial symmetry, any plane through the central axis gives two identical halves — the principal symmetry of diploblastic animals), and a five-armed starfish shown from above illustrating pentamerous radial symmetry, in which the triploblastic echinoderm body is organised into five equal segments aroun …
What this figure shows. An oval comb-jelly (Pleurobrachia) body with two tentacles trailing from opposite sides, annotated to show only two planes of symmetry — one through the longitudinal and sagittal axis, another through the longitudinal and transverse axis — illustrating biradial symmetry as a hybrid of radial and bilateral symmetry, unique to cteno …
What this figure shows. A dorsal view of an insect body showing a single plane down the midline (anterior-posterior axis) dividing it into two mirror-image halves, with a distinct head bearing concentrated sensory structures (cephalisation), clear dorsal and ventral surfaces, and right/left sides — the symmetry pattern shared by all triploblastic bilaterian phyla from flatworms …