Cystoids

Part of speech: noun

Definitions

1. A group of extinct marine echinoderms characterized by their spherical, sometimes spiny bodies and a unique skeletal structure made of calcareous plates
2. An order of ancient sea creatures known for their bladder-like shape and distinctive features, which included a hard outer covering of fused plates
3. An extinct class of echinoderms recognized for their globular forms and complex internal structures, primarily found in fossil records from the Paleozoic era

Etymology: Cystoids are the plural of "cystoid", which is formed from "cyst" and the suffix "-oid", meaning "resembling". The "cyst" element comes from Greek "kystis", meaning bladder or sac, so a cystoid is something shaped like a little sac or bladder. The name fits the group well, since these extinct echinoderms had rounded, bladder-like bodies rather than the star-shaped forms of their relatives. Because the word is a simple combination of a familiar root and a suffix, its history is largely a matter of naming a fossil shape with an existing scientific element. The "-oid" ending, borrowed from Greek "-eidēs" meaning "like" or "having the form of", is productive in scientific vocabulary, and attaching it to "cyst" produced a descriptive label for these creatures.