Actinotroch larva from plankton

On April 13th 2011, our class collected plankton off the F dock in the Charleston marina. I was lucky enough to find an actinotroch larva pictured here. These photos show the same larva in lateral aspect in two different focal planes to emphasize different structures. The first photo is focused on the surface structures, the tentacles, in particular. The second photo is focused on the digestive tract. The larval anterior end is up. The actinotroch is a planktotrophic (feeding on plankton) larva of horseshoe worms (phylum Phoronida). This larva is characterized by a pre-oral hood (at the anterior end), a crown of tentacles posterior to the mouth, and a telotroch, a ring of long cilia at the posterior end. The larval tentacles are paired and located around the middle of the larva, the younger shorter ones near the dorsal midline, and the longest ones near ventral midline. These ciliated tentacles aid in swimming and feeding. The powerful telotroch consists of fused cilia and is used for locomotion. The preoral hood is fringed by a ciliated band. The hood plays a role in the transport of food to the mouth, and is distinctive of actinotrocha. The digestive tract of the actinotroch larva is relatively simple. The mouth located under the hood opens into the spacious stomach, the large oval shape that occupies much of the larval body. Stomach opens into the narrow cylindrical hindgut, which opens to the outside via an anus at the posterior end. After some weeks in the plankton the actinotroch metamorphoses into an adult sessile phoronid worm.