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.

Detailed explanation-2: -Echinoderm larvae are bilateral during their early development. During embryonic development of starfish and sea urchins, the position and the developmental sequence of each arm are fixed, implying an auxological anterior/posterior axis.

Detailed explanation-3: -Echinoderms have a spiny endoskeleton. They have radial symmetry as adults but bilateral symmetry as larvae.

Detailed explanation-4: -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-5: -Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet. These are the only echinoderms that demonstrate “functional” bilateral symmetry as adults, because the uniquely extended oral-aboral axis compels the animal to lie horizontally rather than stand vertically.