On May 7th, I was able to see the end product of metamorphosis of the sand dollar Dendraster excentricus in my culture. It was so exciting to finally see a juvenile sand dollar after watching it go through different stages of development starting from fertilization (at the end of March), and be able to photograph and document the transformation. This first photo shows the secondary podia (tube feet) quite well. They are the longer, more transparent “limbs” coming off the body with cross-containing circles at the tips. At this stage and from this angle, it is more difficult to see the primary podia. Tube feet are used primarily for movement and attachment.
The spines of the sand dollar are the darker “limbs” with no circles at the tips. There are large and small spines, which are homologous to the interambulacral and ambulacral spines in sea stars. The sand dollar juveniles also have two extra large spines marking the posterior end of the animal. These extra long spines can be seen more clearly in the two dark-field photos at the bottom right portion of the juvenile urchin. The middle photo shows the spines of the sand dollar clearly. In the bottom photo, one can see black network-like pigment cells on the aboral (the opposite of oral, which is facing down) side of the juvenile.