These pictures show an echinopluteus larva of the common Pacific coast species - the sea urchin Strongylocentrotus purpuratus (the purple sea urchin). I found this larva in a plankton sample collected off the docks in Charleston, OR on February 8th, 2012. All three pictures show the same specimen at the same developmental stage, but different focal planes, to emphasize different structures. The calcareous spicules making up the larval skeleton are clearly visible in the top photograph and appear rainbow-colored due to the use of polarized light. These spicules characterize the pluteus larva (found in echinoids and ophiuroids, a.k.a. sea urchins and brittle stars). The morphology of the larval skeleton is used to identify larval echinoids (sea urchins and sand dollars). The spicules support the larval arms (4 in this larva at this stage), which in turn support the ciliated band, used for feeding and locomotion. The longer the arms, the longer the ciliated band, the more efficient the larva can feed.
The middle photograph shows the mouth - the large oval shape at the anterior end of the larva (up) and a portion of the tri-partite gut, which characterizes all feeding echinoderm larvae. One can see the round stomach (in the center of the larva) and the esophagus (a short muscular tube that connects the mouth to the stomach). One can also see the two coelomic sacks (one on either side of the esophagus), which will form the body cavity and the water-vascular system of the adult urchin. The bottom photograph shows red pigment cells in the epidermis of the larva. These have been observed to function in wound healing in echinoid larvae: pigment cells in vicinity of the wound migrate to the damaged area and engulf cellular debris (George von Dassow, personal communication).