Most free-living polychaetes rely on a cryptic habit of life for protection from predators, living under rocks, among mussels, or in their tubes during the day, and coming out to feed mostlly at night.  Large jaws are possessed by some species, thought to be used in defense and/or in resolving territorial disputes.

A scaleworm Arctonoe fragilis crawls among the
pedicellariae of a sea star Orthasterias koehleri. Note
that all the pedicellariae are quiescent. This raises the
question as to what stops the worm from being bitten by its
host's pedicellariae? Or is the relationship a mutualism? 3X

Sandworms Nereis spp. have large pincer-
type jaws that they employ in territorial
disputes and in defense from predators

Of 2 common species of sandworms eaten by predatory nemerteans, Nereis vexillosa seems much the tougher prey than Patynereis bicanaliculata.  When being wrapped up by the proboscis of the nemertean Paranemertes peregrina, for example, Nereis bites aggressively with its jaws, flips its hind end and crawls or, in extreme instances, swims away from danger.  Other non-polychaete-eating nemerteans, including Emplectonema gracile, elicit no swimming response in nereid polychaetes.  Roe 1970 Biol Bull 139: 80.

Polychaete Nereis vexillosa being attacked by a nemertean
Paranemertes peregrina. This species of nemertean is
commonly active during periods of low tide, which might
explain why the polychaete does not appear to be protecting
itself...i.e., it is out of water.. In fact, the encounter ends with
the nemertean giving up the attack and wandering away. Photograph
courtesy Rebecca Kordas, Biodiversity Research Centre, UBC, Vancouver.

Predators of adult sandworms include fishes, birds, crabs, and nemerteans, just to name a few.   Studies in Grays Harbor, Washington show that summer diets of staghorn sculpins Leptocottus armatus have a large component of sand worms Nereis brandti, representing about 30% of the fishes' diets based on Index of Relative Importance. Armstrong et al. 1995 Fish Bull 93: 456.

NOTE  this index takes into account both number and size of prey consumed