Here's why:

* Gastrovascular Cavity: This is a single opening that serves as both a mouth and an anus. It's a simple digestive system where food enters and waste exits through the same opening.

* Cnidarians: This phylum includes jellyfish, corals, sea anemones, and hydras. They are characterized by having a sac-like body with a gastrovascular cavity.

Other examples of animals with gastrovascular cavities:

* Platyhelminthes (flatworms): While they don't have a true gastrovascular cavity, they have a branched gut with one opening that acts similarly.

Let me know if you'd like to know more about cnidarians or their gastrovascular cavities!