Flatworm

A flatworm (ヒラムシ, Hiramushi) is a type of deep sea creature first found in.

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

"Flatworms are flat creatures, and while some types look like Sea Slugs, they have little else in common. Some types are venomous and produce a poison similar to the kind blowfish are famous for. Flatworms are hermaphrodites and mate by fighting to determine the loser, who then bears the eggs."

Further information
Flatforms 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.