n. pl.1.(Zool.) An order of irregular sea urchins, usually having a more or less heart-shaped shell with four or five petal-like ambulacra above. The mouth is edentulous and situated anteriorly, on the under side.
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The presence of a posterior process and posterodorsal arms makes the larva of Aspidodiadema jacobyi much more similar to larvae of irregular urchins in the order Spatangoidea than to other families of the order Diadematoida, to which the family i s normally assigned.