How do regular sea urchins differ from irregular sea urchins?

In the phylum Echinodermata, sea urchins (class Echinoidea) are categorized as either "regular" or "irregular" based on their body shape, symmetry, and feeding habits.12

Regular Sea Urchins

Regular echinoids are characterized by a roughly spherical, globular shape. They exhibit a high degree of radial symmetry, with a centrally located mouth on the underside of the body and the anus positioned at the opposite (aboral) pole. These urchins are typically omnivorous or vegetarian browsers, using a complex dental apparatus known as Aristotle’s lantern to scrape algae and other organisms from hard surfaces like rocks.234

Irregular Sea Urchins

Irregular echinoids, such as heart urchins and sand dollars, differ significantly in form and function:12
* Shape: They are typically elongated or flattened rather than spherical.2
* Symmetry: Their body plan deviates from the strict radial symmetry of regular urchins.2
* Anatomy: The position of the anus is shifted to the oral or aboral surface, reflecting a move toward bilateral symmetry.2
* Feeding: Most irregular urchins lack the teeth found in regular species; they are generally burrowers that use their spines and tube feet to collect small food particles from the sediment.4