How many ways can you cut it in half?

Rotate a cutting plane through an animal and see whether the halves match; count how many planes work for sponge, Hydra, earthworm and a star fish larva vs adult.

Find every plane that halves the adult star fish, then show the larva has only one.

Echinoderms are bilateral at every stage.

• Sponges are mostly asymmetrical: no plane through the centre cuts them into two equal halves.
• Radial symmetry: any plane passing through the central axis gives two identical halves. Coelenterates, ctenophores and adult echinoderms are built this way.
• Bilateral symmetry: only one plane divides the body into identical left and right halves, as in annelids and arthropods.
• Diploblastic animals develop from two embryonic layers, an outer ectoderm and an inner endoderm, with an undifferentiated mesoglea between them. Coelenterates are the example.
• Triploblastic animals have a third germ layer, the mesoderm, between ectoderm and endoderm. Every phylum from Platyhelminthes to Chordata is triploblastic.
• Ctenophores, like cnidarians, are diploblastic; the diploblastic phyla are the two with radial symmetry and tissue-level organisation.

Teaching this idea? Put the simulation on your own page or LMS with this code.