Echinoderms begin life as bilaterally symmetrical larva.
Even very different-looking members of this group, like sea urchins and brittle stars, have similar larvae. However, after a few days the larva begins to develop the unique five-fold radial symmetry of this phylum. Sea cucumbers, heart urchins, and sand dollars, however, after passing through a brief stage during which they are radially symmetrical, develop bilateral symmetry again. These Echinoderms retain evidence of their five-part construction into adulthood, although sometimes you have to dissect sea cucumbers to see it. One group of sea cucumbers, the Rhopalodinidae, as discussed by Alex Kerr and Junhyong Kim, continue development past bilateral symmetry back to radial symmetry.