TYLER, S.*; HOOGE, M.; Univ. of Maine, Orono: Mouthless but proboscis-bearing mystery acoelomorph flatworm

Flagellophora apelti (Nemertodermatida, Acoelomorpha, Platyhelminthes) is remarkable for the range of habitats in which it lives (coarse to fine sands and mud at depths from 3 m to 400 m) and the mystery of its means for feeding. Despite having a well-developed gut and a complicated proboscis seemingly well-adapted to capturing food, it lacks a mouth. We have applied transmission electron (EM) and confocal microscopy to verify the mouthless nature and to elucidate the functional morphology of the proboscis. With staining for muscles, we could find no opening in the body wall besides the gonopores and proboscis pore, and even by EM we could find neither a mouth in the body wall nor a connection between the proboscis and the gut. The proboscis comprises elongate gland necks, fully 80% the length of the animal’s body, that can be unfolded through a muscle-enwrapped, epithelium-lined canal and pore at the anterior tip of the body. Powerful retractor and protractor muscles attach to the epithelial cells at a bulbous base through which the necks emerge. At rest, the base sits well behind the statocyst and brain; when the proboscis is extended, the base shifts forward to protrude slightly through the pore, and the gland necks fan out to expose slighlty bulbous, presumably adhesive tips. The cells of the gut contain a single type of unicellular alga in various stages of digestion in vacuoles.