It's a small (ca. 7mm) parasitic Flatworm (Plathelminthes), that lives parasiticly on gills of small fish and eats their blood.
Nothing real special so far, I have to admit.
But now have a look at the picture again. Noticed anything? It's symmetric! And that is the point of this animal: It's actually two animals. And that is not a coincidence, this animal exists in its grown up form only pairwise. When a larvae comes out of her egg, it lives in a free swimming form for a while and then sticks herself to a gill of a fish. If another Diplozoon comes around they start to mature and grow together, if not it will never reach the adult form.
When grown up, the two animals stay together for the rest of their life and continuously fertilize each other. In doing so their genitalia merge, allowing only the sperm of their partner to reach their oocytes. Thus they are the only known completly monogam species.
Isn't that romantic?