Giant Acorn Worm is an undiscovered marine invertebrate of the Eastern North Atlantic and North Pacific Oceans. Adult Acorn worms (Class Enteropneusta) of the type Planctosphaera pelagica have never been observed. The larvae (tornariae) are larger than those of other hemichordates, and if the size ratio is the same as in other species, the adults could grow to 9 feet long. The larvae are large, transparent spheres with arborescently branched, ciliated feeding bands and a U-shaped alimentary tract. Habitat: Oceanic mud at depths of 250– 1,660 feet.
- The larvae may be the abnormally enlarged tornariae of another family of Acorn worms (Ptychoderidae) that fail to metamorphose into adults.