Echinozoan

Echinozoan is a defunct superclass of marine invertebrates that historically encompassed two extant phyla: Echinodermata and Kinorhyncha. The name itself derives from the Greek word 'echinos', meaning hedgehog, and 'zoon', meaning animal, referencing the spiny appearance of some echinoderms. This classification was based on perceived similarities in their larval development and body plan.

Within this grouping, the phylum Echinodermata includes familiar marine animals such as starfish, sea urchins, and sea cucumbers. These organisms are characterized by radial symmetry, typically five-fold, and a unique water vascular system used for locomotion and feeding. The phylum Kinorhyncha, on the other hand, comprises tiny, segmented, worm-like creatures called mud dragons. They possess a distinct head with introvert structures and live in marine and brackish environments.

However, modern molecular and morphological studies have revealed that the echinozoan superclass is paraphyletic, meaning it does not include all descendants of a common ancestor. Echinoderms and kinorhynchs are now understood to be distantly related, with kinorhynchs being more closely allied with arthropods and other ecdysozoans. Consequently, the superclass Echinozoa is no longer considered a valid taxonomic grouping in contemporary biology. Despite its invalidity, the term may still appear in older literature or historical discussions of invertebrate classification.