GENERAL HISTOLOGY

Detailed explanation-1: -Echinoderms take many forms of symmetry. Pentameral symmetry is the major form and the other forms are derived from it. However, the ancestors of echinoderms, which originated from Cambrian period, were believed to be bilaterians. Echinoderm larvae are bilateral during their early development.

Detailed explanation-2: -Eggs hatch into larvae that have bilateral symmetry and can swim. The larvae undergo metamorphosis to change into the adult form. During metamorphosis, their bilateral symmetry changes to radial symmetry. Examples of echinoderms are starfish, sea-cucumber, sea urchins, etc.

Detailed explanation-3: -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.

Detailed explanation-4: -These colorful animals are radially symmetric as adults-they usually have five arms, sometimes more-but as larvae they are bilaterally symmetric like humans. The sea star larvae’s mirror-image symmetry is established when they are egg cells, called oocytes.