What is "saccocirridae"

The Saccocirridae are small interstitial polychaetes common in coarse sand on reflective, surf beaches, usually within the zone of retention. The Saccociridae are members of the clade Protodrilida, which is in turn part of the clade Canalipalpata. Saccocirridae have a world wide distribution and many more species likely remain to be described. These polychates are usually between 2 and 10 mm in length and 500 μm wide. They have reduced parapodia and are considered a true interstitial species, incapable of burrowing through finer sediments.