The balloon worm (Poeobius meseres) is a unique, gelatinous marine worm found in the midwater (mesopelagic and bathypelagic zones) of the ocean, typically at depths of 300-2,500 meters (980-8,200 feet). The balloon worm (Poeobius meseres) is a specialized marine worm that, despite its gelatinous, unsegmented appearance, belongs to the phylum of segmented worms (Annelida). It is a holopelagic species, meaning it spends its entire life cycle floating in the midwater column (mesopelagic and bathypelagic zones) rather than living on the seafloor.