GENERAL HISTOLOGY

Detailed explanation-1: -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-2: -Sea urchins belong to the class Echinoidea, named for the movable spines projecting from their body like a hedgehog’s spines (from the Greek word echinoid meaning like a hedgehog). Sea urchins (Fig. 3.83 A) are common around the world, from the ocean’s shoreline to great depths and from tropical waters to polar waters.

Detailed explanation-3: -The class Echinoidea includes over 900 species of sea urchins and sand dollars. These echinoderms have a hard outer shell called a test that supports spines and tube feet while protecting the internal organs. Sand dollars have short spines, while sea urchins have longer, sharper spines.