In addition to the well-known pentameral symmetry of members of the major living echinoderm classes, the peristomal region of regular echinoids bears the distinctive Aristotle's Lantern, a pentamerous jaw consisting of five calcareous teeth. Irregular echinoids such as the heart urchins do not develop Aristotle's Lantern, and show more bilaterally symmetric as opposed to pentamerous body form.

Thank you Mark. Do you think 5-fold symmetry preceded bilateral in evolution?

That is an interesting question. 5-fold symmetry and bilateral symmetry can alternate in echinoderm lineages, as in the transition from bilateral embryo, to pentamerous adult, to secondary bilateral form as in the irregular echinoids. Some worm phyla such as priapulids show a core bilateral symmetry with radial symmetries superimposed. Pentameral symmetry as seen in the ambulacral grooves of a crinoid is well suited for securing dispersed food sources by means of suspension feeding. If the earliest animals were filter feeders, not unlikely considering their affinity to choanoflagellates, pentamerous body form may have appeared very early and might even have preceded bilateral body form.