Practice question

Class Echinoidea comprises sea urchins, sand dollars, sea biscuits and heart urchins, distinguished by compact globular, flattened or irregular bodies enclosed within rigid corona test of tightly fused calcium carbonate plates, entirely lacking elongated arms projecting from central disc seen in Asteroidea starfish, Ophiuroidea brittle stars and Crinoidea sea lilies. Locomotion uses podia emerging via pores, spines provide defense, and Aristotle's lantern serves for grazing. Reduction of arms correlates with evolution of test, infaunal burrowing in irregular echinoids and epifaunal grazing niche, yet pentaradial symmetry persists internally.