Sea squirt test cells (0.0004 inches thick) are a layer of cells just on the surface the egg, protecting the egg from UV light. This 10 micron thick cell cuts out 90% of the UV that would have hit the egg and damaged its DNA. The follicle cells, seen as the outer layer of cells in the images to the right, are UV transparent and offer no protection from UV, BUT offer mechanical protection from being bumped around in the surf zone.