This parasite is Cryptocotyle lingua. It is a trematode that develops to maturity in seals and other fish eating mammals. It is found in the liver and intestines of seals.
Stunkard, H.W. and Willey, C.H. (1929). Development of Cryptocotyle in its final host. The American Journal of Tropical Medicine, Volume IX 123-125.
Levsen, A., Lunestad, B., Berland, B. (2008) Parasites in farmed fish and fishery products-Fish parasites and consumer health considerations. Improving farmed fish quality and safety, 17.4 441-443.