Practice question

Echinoderm endoskeleton is mesodermal, not epidermal exoskeleton, composed of stereom, a distinctive porous three-dimensional lattice of calcium carbonate predominantly as calcite with small magnesium carbonate. Mesenchymal sclerocytes secrete interconnected ossicles forming articulated arms in sea stars, rigid test with fused plates in sea urchins, and scattered spicules in sea cucumbers. Thin epidermis covers ossicles, spines articulate via muscles. This calcareous construction provides protection, muscle attachment for tube feet and support, contrasting with chitinous arthropod cuticle and siliceous sponge spicules, thus serving as diagnostic phylum character for Echinodermata identification.