The polychaete worm Owenia collaris has a beautiful mitraria larval stage that we have been able to culture in lab. The following pictures show a week-old mitraria larva oriented with its anterior end up. The mitraria larva is transparent and has a through gut, a long equatorial ciliated band, and a bundle of very long larval chaetae (bristles).
The first picture taken at lower magnification shows the general shape of the larva, the entire digestive tract, the ciliated band (golden-brown ring that spans the broadest portion of the larval body) and the chaetae. The large brownish oval in the center of the larval body is the stomach, which is connected to the pink intestine (hindgut), on one side, and the greenish esophagus (foregut) on the other. The mouth opens below the ciliated band (on the right in this picture) and leads into the foregut. The intestine opens via the anus below the ciliated band near the center of the larval body.
The middle picture is a close-up view, highlighting some of the internal features such as the stomach and the foregut. The mitraria is a planktotrophic larva, and we have been feeding it unicellular algae, Rhodomonas and Dunaliella, whose pigments color the larval gut. The apical organ can be seen at the apex of the larval body from which a tuft extends.
The bottom picture is a ventral view of the larva, which showcases the paired chaetal sacs from which the chaetae protrude. This image also shows the round pink anus just anterior to the chaetal sacs. In the next three weeks or so, this larva will develop an invagination between the mouth and anus, which will form the trunk of the juvenile worm!