There is also evidence for the importance of actin-based structures in epiboly. Treating embryos with the actin destabilizer cytochalasin b results in delayed or arrested epiboly. Gastrulation ends when epiboly of the hypoblasts closes the blastopore. Completion of epiboly requires the coordination of cytoskeletal changes across the embryo. The gastrulation in annelids occurs by epiboly and in Balanoglossus by emboly. "Elysia chlorotica" gastrulation is by epiboly&colon; the ectoderm spreads to envelope the mesoderm and endoderm. The EVL also moves vegetally during epiboly, increasing its surface area as it spreads. The EVL, YSL, and DEL all undergo epiboly.