Planktonic larvae of the sand dollar, Dendraster excentricus, can remain in the water column for various amounts of time from a few weeks up to two months (Emlet, 1986). They have cilia, that help them feed and move in the water. These pictures show six-armed pluteus larva of D. excentricus from ventral side (where the mouth opens). Larval mouth is facing us. It is surrounded by a circumoral (= around the mouth) ciliated band, stretched out on the larval arms. The cilia in the ciliated band direct food, such as microscopic algal cells, into the mouth. The top picture shows the post-oral (= posterior to the mouth) portion of the ciliated band stretched between the two post-oral larval arms. The second picture is of the same larva, but in a different focal plane, showing the pre-oral portion (anterior to the mouth) of the ciliated band. The third picture shows the same larva, in a different (deeper) focal plane. I am now focussing on the larval gut. From the mouth the food particles are directed into the esophagus (the anterior portion of the larval gut). The mouth (upper left) and esophagus together make up the bulb-like shape. Mouth is the “head” of the bulb, and esophagus is the narrower portion.
The esophagus is surrounded by a layer of circular muscles. Peristaltic constrictions of these muscles force the food particles toward the stomach (the middle portion of the larval gut, separated from the esophagus by a cardiac sphincter). Musculature in the esophagus helps open the cardiac sphincter to allow food to enter the stomach (Burke, 1981). The stomach is the large oval shape occupying majority of the space inside the body of the pluteus larva. The sphincter is the lentil-like shape between the esophagus and the stomach.