In our embryology class we have spent a lot of time raising and observing the development of echinoids and asteroids. The first two pictures show the planktotrophic, or feeding on plankton, larvae of these two classes of echinoderms. The top one is the pluteus larva of the sand dollar, Dendraster excentricus. Unlike other echinoderm larvae, the pluteus larva has long arms supported by calcareous skeletal rods, which you can see on this picture. A contiguous circumoral ciliated band spans the arms of the pluteus larva.
bipinnaria larva of the ochre seastar, Pisaster ochraceus. The bipinnaria is characterized by a circumoral ciliary band divided into a pre-oral and post-oral loops, and lacks calcareous spicules. I was amazed at the difference in structure of these two feeding larvae, namely the presence and absence of the spicules. I did not know why these two related organisms had such different larvae. It turns out that this difference has a developmental explanation. Early cleavage in echinoids and asteroids is very similar. However, starting at the 16-cell stage, there is a subtle difference. In asteroids, the division from 8-cell-stage to 16-cell stage is equal, i.e. all 16 cells are of the same size. In most echinoids this division is unequal, i.e. it results in cells of different sizes.
The cells at the animal pole divide equally into eight cells, called mesomeres but the cells at the vegetal pole divide unequally into four large cells, called macromeres, and four small cells, called micromeres. The micromeres give rise to a population of cells in the early gastrula of echinoids, called primary mesenchyme. Primary mesenchyme cells ingress into the blastocoel before the primary gut starts to invaginate. In the third picture you can see the primary mesenchyme cells inside the blastocoel of a gastrula of D. excentricus. The primary mesenchyme cells go on to make the spicules in the pluteus larva. In the starfish the micromeres are absent, and there is no primary mesenchyme. Gastrulation in starfish begins with invagination of the primary gut, as you can see on the bottom picture of the early gastrula of P. ochraceus. The absence of the micromeres and the primary mesenchyme is causally related to the lack of larval spicules in the bipinnaria. That is amazing!