GENERAL HISTOLOGY

Detailed explanation-1: -Asteroidea, or sea stars, are recognizable by their star-shaped bodies and often colorful appearance. They have a hard exoskeleton made up of calcium carbonate plates and a series of flexible arms that radiate out from their central body.

Detailed explanation-2: -Sea Urchins (Echinoidea) Sea urchins and sand dollars are classified within a broader group called the Echinoidea. Broadly speaking all echinoids have an endoskeleton composed of distinct fused plates which often form distinctive patterns.

Detailed explanation-3: -Sea urchins, together with their close relatives the heart urchins and sand dollars, comprise Class Echinoidea within Phylum Echinodermata.

Detailed explanation-4: -Echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea. They have a hard shell (referred to as a test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.

Detailed explanation-5: -Like all echinoderms, echinoids have a skeleton composed of calcitic plates embedded in their skin (their skeleton is internal, like ours). This skeleton has a very particular structure, termed stereom.