GENERAL HISTOLOGY

Detailed explanation-1: -Diversity. There are about 7, 000 extant species of echinoderm as well as about 13, 000 extinct species.

Detailed explanation-2: -Feather stars, sea lilies and starfish fossils are traditionally extremely rare as their multiplate skeletons quickly falls apart after death meaning only instant burial would allow them to be preserved whole.

Detailed explanation-3: -Echinodermata: Systematics. In traditional taxonomy, there are five classes of living echinoderms: Crinoidea (sea lilies), Asteroidea (starfish), Ophiuroidea (brittle stars or snake stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers).

Detailed explanation-4: -Examples of echinoderms include sea stars, sea urchins, sea cucumbers, brittle stars, and feather stars (Fig. 3.83). Although they may appear very different, echinoderms all have two major defining characteristics that set them apart from all other animals: a water vascular system and five-sided radial symmetry.

Detailed explanation-5: -The phylum echinoderms is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers).