Hydrozoans use a structure called a polyp to attach themselves to rocks.

Here's why:

* Polyp: A polyp is the sessile, tubular body form of a cnidarian (like a hydrozoan). It has a mouth surrounded by tentacles, and a basal disc that attaches it to a substrate, like a rock.

* Hydroids: Hydrozoans often exist in two body forms: polyps and medusae. The polyp form is responsible for attaching to surfaces.

So, when you see a hydrozoan clinging to a rock, it's likely the polyp form of the animal.