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.