Last week in my embryology class, I took some pictures of the starfish bipinnaria larvae (Evasterias troschelii). Like other planktotrophic echinoderm larvae, bipinnaria has a complete tri-partite gut. The mouth is a clear rounded triangular shape in the anterior third of the animal (upper left on this photo). The mouth leads to the esophagus (a wide tube below the mouth), which connects to the stomach. I found it interesting that I was able to see some color in the larval gut, which comes from the unicellular algae that we feed to the larvae. The large pinkish egg-like shape in the posterior third of the animal (bottom right) is the larval stomach. The elongated shapes on either side of the esophagus and stomach are the larval coeloms. The left coelom is going to make up the majority of the water-vascular system in the adult starfish. The dark ribbon-like shape is the larval ciliated band (out of focus on this picture).
This picture shows the same larva in the same orientation, only with different parts in focus. It is easy to see the intestine (narrow tube in the posterior third of the larva) with the same red pigment as in the stomach, only darker! The intestine curves up and leads to the anus, located on the ventral side (in focus here). These larvae are able to swim and feed using a ciliated band composed of many tightly opposed epithelial cells, each with a cilium that looks like a little hair. The cilia (plural of cilium) beat continuously to create a water current away from the mouth, and locally redirect the flow toward the mouth, when they encounter food particles. One cannot distinguish individual cilia on this photo, however the portion of the ciliated band that flanks the mouth above and below is distinguishable and sharply in focus. The portion of the band that is in focus above the anus is called "postoral", and the portion of the band above the mouth is called "preoral". During development of the bipinnaria larva the preoral and postrodal portions of the ciliated band separate to form two separate loops (preoral and postoral).