Flatworm

The Flatworm (ヒラムシ, Hiramushi) is a type of sea creature in the introduced in.

In
While initially absent from, the Flatworm was added in the 1.3.0 Free Summer Update.

In
Upon donating a Flatworm to the museum, it can be found in the large pool in the first room of the sea exhibit, along with many other species of sea creatures. It is found near the entrance of the exhibit. The exhibit has this to say about the Flatworm:

In
When donating to the museum, Blathers will say the following:

Once donated, the flatworm can be found in the coastal tank, alongside the Seaweed and the Abalone.

Real-world information
Flatworms are their own phylum, which includes tapeworms and flukes. Over half of all known flatworm species have parasitic behavior, which means that they thrive on another organism, a host, at the host's expense. Flatforms also lack a distinguished respiratory and circulatory organs, which is one reason for their flat shape. As a result, flatworms must rely on diffusion to take in oxygen while expelling carbon dioxide.