These are Acanthocephalans or thorny headed worms. They use their proboscis (shown below) to burrow into the intestine. The hooks on the proboscis help them to pierce and attach to the intestinal mucosa. These hooks also serve as a way to differentiate between species. The number and size of hooks as well as the size of the proboscis varies between species.
Pictured is Corynosoma wegeneri, the species found in local pinnipeds.
Amin, Omar M. (1998) Marine Flora and Fauna of eastern United States, Acanthocephala, 1-5.
Raja, Juan Antonio. Parasite Diversity. Parasites (Marine Mammals), 5. http://what-when-how.com/marine-mammals/parasites-marine-mammals/.