Here’s a personal view about body symmetry and body openings from someone who lived through the evolution of bilateral symmetry.
Form? I didn’t have any; that is, I didn’t know I had one, or rather I didn’t know you could have one. I grew more or less on all sides, at random; if this is what you call radial symmetry, I suppose I had radial symmetry, but to tell you the truth I never paid any attention to it. Why should I have grown more on one side than on the other? I had no eyes, no head, no part of the body that was different from any other part; now I try to persuade myself that the two holes I had were a mouth and an anus, and that I therefore already had my bilateral symmetry, just like the trilobites and the rest of you, but in my memory I really can’t tell those holes apart, I passed stuff from whatever side I felt like, inside or outside was the same, differences and repugnances came along much later.Excerpt from The Spiral, a tale in the delightful Cosmicomics collection of science-inspired short stories by Italo Calvino.
Every time I open Spotify, I see the pattern of engrailed expression in an early Platynereis larva. Once you see it, there is no turning back!
Prud’homme, B., de Rosa, R., Arendt, D., Julien, J.-F., Pajaziti, R., Dorresteijn, A. W. C., Adoutte, A., Wittbrodt, J., & Balavoine, G. (2003). Arthropod-like expression patterns of engrailed and wingless in the annelid Platynereis dumerilii suggest a role in segment formation. Current Biology: CB, 13(21), 1876–1881. https://doi.org/10.1016/j.cub.2003.10.006
This is a bryozoan embryo exhibiting its blastopore. These animals are discreet but ubiquitous in oceans and lakes all over the world.
What we see is the DNA inside the nucleus of the cells of the embryo. The color gradient indicates if the nuclei are closer (yellow) or further away (purple) from the microscope camera.
The embryonic cells are arranged in a circle and form a central opening that we call the blastopore. This opening, in bryozoans, will become the mouth of the animal after the embryo develops.
What about our mouth, where does it come from?
Live footage of entoprocts! Tiny colonial invertebrates that capture food with a crown of ciliated tentacles