Craspedus

A craspedus is a type of jellyfish belonging to the subclass Craspedota of the class Hydrozoa. The defining characteristic of members of this subclass is the presence of a velum, a shelf-like extension of the bell margin that aids in propulsion. Craspedota encompasses both hydromedusae (the free-swimming medusa stage of hydrozoans) and a small number of hydrozoan polyps. The presence of the velum distinguishes them from other jellyfish forms, such as those found in the Scyphozoa class, which lack this structure. The subclass Craspedota is often contrasted with Aphanerota, which includes hydroids without a medusa stage or those with a sessile, degenerate medusa. The taxonomy of Craspedota and its related classifications within Hydrozoa have been subject to revision and debate within the scientific community.