Echinus rudiment formation

Shown here is the echinopluteus larva of the purple sea urchin, Stronglyocentrotus purpuratus (top image), and a sand dollar Dendraster excentricus (bottom image). Larval anterior is up. In both pictures, you can see the juvenile sea urchin or sand dollar developing inside the larval body. The rudiment of the juvenile, called the echinus rudiment develops in a pouch called a vestibule, which forms as an unpaired epidermal invagination on the left side of the larval body. The left coelomic sac contributes to the formation of the juvenile. The top picture shows the three-week old larva of S. purpuratus from the ventral side. This larva does not yet have a well-developed echinus rudiment, but the invagination can be seen clearly to the right and above the stomach, which is the large dark oval shape in the middle of the larva.

The three-week old D. excentricus larva shown here has a much more advanced juvenile rudiment than the S. purpuratus larva. The larva is shown from ventro-lateral view, so that the juvenile rudiment is facing you. The juvenile rudiment is quite large and takes up most of the posterior portion of the larva, obscuring the larval stomach, which barely peaks out from behind the juvenile rudiment on the left. By the late pluteus stage, the juvenile is almost fully developed within the vestibule, and will have acquired juvenile spines and skeletal structures. The juvenile also has five podia, or tube feet, on the oral side of the juvenile, which is the side facing away from the larval stomach. Juvenile tube feet can be extended out of the vestibule and retracted again. This D. excentricus juvenile has developed podia, which appear as five lobes on the right. When ready to settle, the juvenile extends its podia from the vestibule, contacts the substrate, and walks away, having re-absorbed much of the larval body.