Our class collected plankton on April 12, 2012 using a 153-μm mesh size net towed behind a boat two miles outside Coos Bay, OR. The two pictures show the 0.7 mm long unidentified actinotroch larva of a phoronid worm that I found in this sample.
The top image is focused on the tentacles, each with a single black pigment spot. This larva had 14 tentacles total. At the anterior end (up) one can see the pre-oral hood, and at the posterior - the telotroch, a prominent ciliary band that surrounds the anus and propels the larva. The bottom picture shows the same individual, but focusing deeper in to show the internal structures. At the apex of the pre-oral hood, one can see a thickened region of epithelium - this is the apical sense organ (the larval brain). It may play a role in substrate selection during metamorphosis (Johnson and Zimmer, 2002).
On this picture one see almost the entire digestive tract. The oral hood encloses a funnel-shaped vestibule, at the tip of which is the mouth. The mouth leads into the stomach (a large oval shape that occupies the majority of space in the larval trunk). The stomach connects to the hindgut - a short straight tube which opens at the posterior end via an anus.One can also see a small but distinct mid-ventral (to the right on the picture) invagination just posterior to the tentacles (and next to a pigment granule). It is called the metasomal sac. This sac will grow throughout larval development and eventually wrap around the larval gut. At the onset of metamorphosis the metasomal sac is everted to form the trunk of the adult worm. The entire larval gut is pulled into the everted sac, thus shortening the larval axis, and bringing the mouth and anus closer together to form the U-shaped adult gut.
Johnson, KB and Zimmer, RL. 2002. Phoronida p.430. In: Atlas of Marine Invertebrate Larvae. Edited by C. M. Young. Academic Press. New York.