Utility-worms (Family Opheliidae) are active burrowers, contributing greatly to the bioturbation (aeration) of sediments. They all burrow head down in sand or mud. Their fusiform (cigar) shapes vary dramatically from grub-like forms, such as the pupa utility-worm (AN10), to those with a cylindrical design and enlarged head, like the sword utility-worm (AN11), or several other species that are slender and torpedo-shaped. The torpedo-shaped worms, Armandia species, are amazing swimmers, moving with lateral body undulations similar to that of salmon or sea snakes.

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