AP BIOLOGY

Detailed explanation-1: -The echinoderms have a radially arranged, pentamerous body structure that is very different from the bilateral body structure of the related deuterostome phyla, the hemichordates and the chordates. Yet, all three phyla evolved from a common bilaterally symmetric ancestor (Eernisse & Peterson 2004; Smith et al. 2004).

Detailed explanation-2: -Echinoderms (starfish, sea urchins and their allies) are an exception; all modern species exhibit pentaradial (five-fold) symmetry, but the phylum also possesses an extensive fossil record that includes bilateral, asymmetrical and radial forms [1, 2].

Detailed explanation-3: -To adapt to their benthonic habitat and planktonic habitat niches, echinoderms evolved from bilateral symmetry first to asymmetry, then to pentameral symmetry [9]–[11].

Detailed explanation-4: -One characteristic of an echinoderm is that they all have pentamerous radial symmetry. This simply means they all have 5 equal body parts. You can divide these animals into five equal parts, and every part is the same. For example, most sea stars have five arms or a multiple of five arms.