Unknown Advanced Actinotroch

This picture shows a lateral view of an unknown actinotroch larva of a phoronid worm that was found on May 10th in plankton sample taken off the F Dock in the Large Boat Basin of Charleston, OR.   This picture clearly shows a ring of tentacles (called the lophophore) at the anterior end of the larva (up). The lophophore is used for feeding and each tentacle is ciliated. The oral hood is visible just anterior to the lophophore. It is pulled over the mouth, much like a hood, and is used in feeding as well. This picture also shows the telotroch,  a ciliated ring at the posterior end of the larva which is used for swimming.

The second picture is a ventral view of the same larva. The telotroch, lophophore, and oral hood are clearly visible. It also shows the achtinotroch’s digestive tract. The stomach is the large internal cavity, and the narrow, tapered end toward the postrior is the intestine. The structure that is wrapped around the posterior part of the stomach (partially outlined by yellow pigment granules) is the metasomal sac. Metasomal sac grows inside the larva, is everted at metamorphosis and becomes the trunk of the juvenile. For a view of an early metasomal sac, see an earlier post by Mark Inc.

The third photo is a dorsal view of the larva. The metasomal sac is now on the far side of the stomach, though it is still visible. The anterior-most end of the sac opens to the outside via the metasomal pore. The sac is everted through this pore. The mouth of the larva is visible as a triangular shape at the anterior end, just below a cluster of pigment granules. It is inside the oral vestibule formed by the oral hood.