The Enigmatic cave hydrozoan (Velkovrhia enigmatica) is the only member from the phylum Cnidaria in the world that lives exclusively in caves. The species is endemic to the Dinarides and has so far been recorded in only five localities in Slovenia, Croatia, and Bosnia and Herzegovina. Unfortunately, the species has been recorded more than once in only two localities.
The Enigmatic cave hydrozoan is a colonial organism. It grows like a polyp 1.3 – 1.7 mm in height and has between 5 and 36 weakly contractible tentacles. Polyps in the colony grow from a common stolon and 2 – 5 mm from each other. The species is sessile. It lives in freshwater underground lakes, attached to rocks and cave walls.