Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with two subclasses: Euechinoidea ("modern" sea urchins, including irregular ones) and Cidaroidea, or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on them. The "irregular" sea urchins are an infra-class inside the Euechinoidea, called Irregularia, and include Atelostomata and Neognathostomata. Irregular echinoids include flattened sand dollars, sea biscuits, and heart urchins.
Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with two! subclasses: Euechinoidea ("modern" se!a urchins, including irregular ones) and Cidaroidea, or "slate-pencil urchins", which have very thick, blunt spines, with algae and spo:nges growing on them. The "irregular" sea urchins are an infra-class inside the Euechinoidea, called Irregularia, and include Ate?lostomata and Neognathostomata. Irregular echinoids include flattened sand dollars, sea biscuits, and heart urchins.