Welcome to my small celebration of symmetry. This beach party’s a renewal of kinship with animals we share symmetry with and a reminder that symmetry varies in ways worth appreciating. An in-depth survey of symmetry would expose developmental and evolutionary mysteries better left to zoology and comparative anatomy courses. So instead, let’s head to the shore, where examining a few examples from the world of bilateral animals will serve as a foundation for recognizing and enjoying less familiar layouts.
Most of us are familiar with bilateral symmetry because that’s what people and their most common pets have. A bilateral body can be divided into roughly mirror images along only a single plane. Animals with bilateral symmetry have left and right sides, what we sometimes call handedness.
This cast of characters gets along without handedness. Like pizza, their bodies can be divided into equal parts along a few to many planes.
Ctenophores lean to the biradial. They have eight rows of cilia for locomotion. They appear radial (imagine dividing the body like an eight-piece pizza). But, the analogy falls apart; they have a pair of feeding tentacles originating down by the mouth. With that, I’m on thinner symmetry ice than I like.
A tip of my cap to asymmetry
thoutershores is an inclusive place, so there’s no way I’m celebrating symmetry without inviting the sponges. There’s no plane of symmetry by which sponges can be divided into two equal parts.
My symmetry party illustrates that examples of the main types of animal symmetry abound on the intertidal shore. Variations on the bilateral form are dazzling. There are fast movers among them and sessile forms too. Speaking of sessile forms, how about the radial crowd at the tide pools? Starfishes and anemones aren’t using their pizza pie symmetry for speed. Neither are the pelagic jellies; they’ve got good movement, but they do not speed, and being a bit at the mercy of the currents, they tend to get tossed up on the beach. So head over there. It won’t be long ’til you find a beachcast jelly. While you’re there, keep an eye out for symmetrically enigmatic ctenophores.
I updated this post on September 20, 2022.