Echinoid

Part of speech: noun, adjective

Definitions

1. A spiny marine organism belonging to the echinoderm class This includes species such as sea urchins and sand dollars It exhibits radial symmetry and has a hard calcareous shell found in aquatic environments
2. A sea creature characterized by a rigid, spiny outer skeleton and radial symmetry, part of the echinoderm group
3. An aquatic invertebrate known for its hard, spiked shell and membership in the Echinodermata phylum

Etymology: "Echinoid" is a compound built from two Greek elements. The first, "echino-", comes from Greek "echinos", which meant "hedgehog" or "sea urchin", and the same root gave English "echinus", the rounded moulding beneath a Doric capital. The second, "-oid", comes from Greek "eidos", meaning "form" or "likeness", and turns a noun into "resembling" something: a "humanoid" resembles a human, a "asteroid" resembles a star. Put together, the word means something like "sea-urchin-like", and it names the class of animals that includes sea urchins, sea biscuits and their relatives. The spiny shell that gives these creatures their name is the feature that first made the comparison obvious. Because a sea urchin's body is built on five-fold radial symmetry, the same description also applies to the wider echinoderm group, which is why the term is used for the urchins and their close kin.