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: -Even so, the larvae of all echinoderms are bilaterally symmetrical, and all develop radial symmetry at metamorphosis.

Detailed explanation-3: -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-4: -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-5: -Echinoderm embryos undergo complete radial cleavage and undergo gastrulation by invagination. These gastrulae develop into a bilaterally symmetrical larva, the structure of which differs in each echinoderm class, and within classes dependent on egg size.