GENERAL HISTOLOGY

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: -Although the echinoderms show both radial and bilateral symmetry in their life cycle, they are classified as bilaterally symmetric and not radially symmetric because they are evolved from bilaterally symmetrical organisms.

Detailed explanation-3: -n. Symmetrical about an axis that runs from the oral surface where the mouth is to the side opposite the mouth. Cnidarians have radial symmetry. Echinoderms are bilateral animals that have a 5-part radial symmetry as adults.