- Norwegian: Thecate hydroider
The polyps always live in colonies. Each polyp live in a protective holster, unlike the polyps of the Athecate hydroids. The holster is usually long enough to enable the polyp to fully contract into it.
Thecate hydroids are one of two orders of hydroids. There are approximately 1900, all marine, species in the world today. Another, frequently used name for this order, is Leptomedusae.