GENERAL HISTOLOGY

Detailed explanation-1: -Their bodies look like pin cushions. The sea star’s skeleton, like the sea urchin’s, is an endoskeleton consisting of small plates of calcium carbonate embedded in the epidermis. These plates, called ossicles, are much smaller than those of sea urchins.

Detailed explanation-2: -Sand dollars and sea urchins have no arms. Sand dollars look like coins. Their flat bodies are covered with spines that help them burrow into the sand. Sea urchins look like pincushions or round brushes.

Detailed explanation-3: -Sea urchins (/ˈɜːrtʃɪnz/) are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to 5, 000 meters (16, 000 ft; 2, 700 fathoms).

Detailed explanation-4: -Sea urchins belong to the phylum Echinodermata–the same group as sea stars, sand dollars, sea lilies and sea cucumbers. Although difficult to see through all the spines, sea urchins also have a hard outer body like that of its relatives.