Tidepool Tim says,  “These are the more common scale worm we find on our rocky shorelines.  Scale worms are a segmented annelid worm that is short and fat relatively.  They have loose scales like a fish on their dorsal side and bristly setae as well. Flip a scale worm over and you will see a naked belly with a visible red blood vessel.  Sometimes they shed scales when you are handling them - if they are really stressed they will drop all of their scales.  It appears that they may be able to re-grow new ones - like we re-grow fingernails.   

Scale worms are active hunters feeding on a variety of tide-pool invertebrates, but most other worms.  They have a proboscis that everts with a "clicking sound" the end of which has sharp toothlike teeth. once when I was holding a fistful of scale worms I felt a snap or two from the pile of worms.  Closer inspection revealed that some of the worms were throwing out their beaks or teeth with a resounding snapping noise.  All I could think was that they were agitated, perhaps feeling too warm from the heat of my hand.....who knows.  I panicked and threw them into my collecting pail just to be safe. Pure cowardice some may say....ha ha! Spaghetti worms share their burrows with scale worms on more muddy beaches. Not sure what the arrangement is with the two species, but it must be some sort of mutualism or commensalism. Most of the scale worms we collect for science orders are found on rocky shores near the low tide mark - on spring tides. It's nothing for us to collect 1000 scale worms in an hour.  They are incredibly abundant.”