The first two pictures show a 30-hour old gastrula of the sand dollar Dendraster excentricus. During echinoid gastrulation the first cells to enter the blastocoel (the spatial gel-filled cavity inside the embryo) are called primary mesenchyme cells. Then, the archenteron (the primary gut) buckles in (invaginates). Through a series of cell rearrangements called convergence and extension, the archenteron (a large tube visible here inside the embryo) elongates and extends towards the roof of the blastocoel. In the meantime the primary mesenchyme cells form two groups and begin to secrete calcareous larval spicules (two tri-radiate spicules are visible here on either side of the archenteron). Approximately two thirds of the way up, a second group of cells ingress into the blastocoel from the roof of the archenteron. This is where the filopodia come in.
This image is a close up view of the tip of the archenteron, showing a secondary mesenchyme cell with a long conical projection reaching toward the roof of the blastocoel. These processes attach to the roof of the blastocoel and aid in the final extension of the archenteron.
This picture shows a 62 hour old gastrula of a sea star Pisaster ochraceous. Asteroids lack the primary mesenchyme cells, so the mesenchyme cells that ingress at the tip of the archenteron are the only mesenchyme they have. Note the lack of primary mesenchyme cells in the blastocoel and the numerous branched filopodia on the mesenchyme cells at the tip of the archenteron.