any member of the invertebrate phylum Sipuncula, a group of elongated, often spindle-shaped, unsegmented marine worms. The head usually has one or more rings of tentacles. Peanutworms vary in length from a few to 500 millimetres (1.6 feet) or more in length. Though rare, they may be locally common on seabeds throughout the oceans of the world. Peanutworms are bottom-dwelling (benthic) animals; most burrow in the mud or sand between tide levels or in oozes of the deepest ocean trenches. Some species have other habitats and live in discarded mollusk shells, in sponge siphons, in corals, among the twisted tubes of encrusting polychaetes (marine annelid worms), and even in tangled roots of marine plants.
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