In the northwestern Atlantic, these hydroids are especially common on the outside of shells that are occupied by the flat-clawed hermit crab.

The club- like feeding polyps grow up to thirteen millimetres in length, and have two rows of eight tentacles, the lower set being shorter than the upper ones.

Scattered among these are specialized defensive stinging polyps (dactylozooids) that are long, coiled threads.

The gonozoids liberate crawling planula larvae that search out moving gastropod shells[3] H. echinata is found on all sides of the northern Atlantic Ocean, including the Arctic Ocean, the Saint-Lawrence Gulf, the Baltic Sea and the North Sea south to northwest Africa, as well as the Western Atlantic including the Gulf of Mexico.

[4] It is common around the coasts of Britain and Ireland, and there it is found where hermit crabs (Eupagurus bernhardus) occur, on the lower shore on sandy substrates.