Derogenes varicus (Müller, 1784) belongs to a large group of species known as the Hemiuroidea, many of which also occur in the stomach of fish. The details of the parasites relationship with the fish are as yet unknown.
The parasite may reach about 3mm long and 0.8mm wide.
At the anterior or front end is the mouth (M) surrounded by the oral sucker (OS).
In the middle of the worm is a large attachment sucker, the ventral sucker (VS).
Behind this sucker are sex organs, the two testes (T), the ovary (O) and the two yolk or vitelline glands (V).
There are many eggs (E) spread throughout the body.