In a plankton tow taken in Charleston, OR on February 20, 2013, I found several polychaete nectochaete larvae of Magelona sp., which I was able to identify by their long anterior tentacles. The tentacles apparently develop as extensions of the prototroch (larval ciliary band) and are suspected to be used in locomotion and feeding (Wilson 1982). I noticed the tentacles were relaxed and extended when the larva was hanging suspended in the water column, but during rapid sinusoidal movements of the body, the larva coiled up its tentacles and kept them close to the body.
This picture is a close up of one of the tentacles. You will notice that they are covered by papillae. The papillae are present along the entire length of the tentacle but only on the dorsal side. Pairs of papillae appear to be at right angles to each other as they each emerge from the surface of the tentacle. It is suspected that the papillae may serve a sensory function by detecting vibrations in the surrounding water (Jones 1968).
This is a close up of the larval mouth (ventral view) - note its distinct triangular shape. Two eyespots are also noticeable near the anterior end of the larva. Toward the posterior (down), bundles of bristles emerge from paired chaetal sacs on each segment of the body.
Wilson DP. 1982. The larval development of three species of Magelona (Polychaeta) from localities near Plymouth. J Mar Biol Ass UK 62: 385-401
Jones ML. 1968. On the morphology, feeding, and behavior of Magelona sp. Biol Bull 134(2): 272-297.