Cnidarians are a phylum of aquatic invertebrates that include:

* Jellyfish: The most well-known cnidarians.

* Corals: These are colonial animals that form large reefs.

* Sea anemones: Sessile animals that resemble flowers.

* Hydras: Tiny freshwater polyps.

The name "cup animal" comes from the polyp body form of cnidarians, which is shaped like a vase or cup. The polyp is the sessile, attached form of the cnidarian, while the other form, the medusa, is the free-swimming, jellyfish-like form.