Summary
Animals show radial, bilateral or no symmetry, which relates to their lifestyle and movement.
In depth
Radial symmetry (cnidarians, adult echinoderms) suits sessile or drifting animals sensing from all sides; bilateral symmetry allows cephalisation (concentrated head and sensory organs) and directed movement. Asymmetry is seen in sponges.
Bilateral symmetry allows directional movement and head development. Radial symmetry suits sessile life. Spherical symmetry occurs in some protozoa.
Example: starfish adults are radial, larvae bilateral.
Asymmetry occurs in sponges, and spherical symmetry in some protists. Bilateral animals are cephalised, with sensory organs and neurons concentrated at the head end, which supports directed movement, hunting and complex behaviour. Radial animals meet the environment from all directions, which suits a sessile or floating lifestyle. Biradial symmetry, in comb jellies, is in between.
Symmetry is established in the embryo by gradients and genes (Hox, Wnt, BMP). Flatfish such as flounder are bilateral as larvae, with one eye migrating to the other side to become asymmetric as adults. Heart and gut positions in vertebrates are asymmetric inside a bilateral body, determined by cilia flow in the embryo.
Example: starfish have radial adults but bilateral larvae, which shows their ancestry.
Frequently asked questions
What is Animal Symmetry?
Animals show radial, bilateral or no symmetry, which relates to their lifestyle and movement.
Which area of biology covers Animal Symmetry, and how advanced is it?
Animal Symmetry is one of the 53 topics in Zoology & Animal Diversity. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Germ Layers, Echinoderms, Vertebrates and Invertebrates.
Can you give an example of Animal Symmetry?
Flatfish larvae are bilateral but adults become asymmetric.
What should I learn before Animal Symmetry?
Work through the Beginner topics in Zoology & Animal Diversity first, such as Animal Kingdom, Vertebrates and Invertebrates, Sponges, then return to Animal Symmetry. Intermediate topics assume the Beginner vocabulary.
What topics are related to Animal Symmetry?
The closest topics to Animal Symmetry are Germ Layers, Echinoderms, Vertebrates and Invertebrates, Animal Kingdom. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Animal Symmetry?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Animal Symmetry. To practise the whole of Zoology & Animal Diversity, use the Zoology & Animal Diversity quiz.
How is Animal Symmetry different from Germ Layers?
Animal Symmetry: Animals show radial, bilateral or no symmetry, which relates to their lifestyle and movement. Germ Layers: Ectoderm, mesoderm and endoderm give rise to all animal tissues; simple animals have only two. Read both side by side and note what each one explains.
How should I study Animal Symmetry step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Germ Layers and explain the difference aloud. Finally, take the Zoology & Animal Diversity quiz and come back to this page for any question you miss.