Nemertean worm - Malacobdella grossa

Apparently not appreciated by taxonomists

These nemertean worms are found in razor clams (Siliqua patula) though are also common to less harvested clams such as White sand clams (Macoma secta) and Bent nose clams (M. nasuta)

The worm is thought to be commensally parasitic (i.e. does not negatively affect its host) with clams and snails living in their mantles and eating plankton. Ironically, this “parasite” hosts its own variety of parasites including Haplosporidium malacobdellae, a sporozoan, which can cause castration or death to the worm. (Morris et al. 1990)

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References:

Morris, Robert H., Donald P. Abbot, and Eugene C. Haderlie, 1980, 1992.  Intertidal Invertebrates of California.  Stanford University Press, Stanford, CA. 88. 690 pp.