Abstract Ascidian test cells co-differentiate on the surface of each ovarian oocyte beneath the vitelline coat. They become vacuolated and later occupy the perivitelline compartment of each egg and embryo. In some species, their vacuoles contain submicroscopic granules or filaments called ‘ornaments’ and acidic glycosaminoglycans. These test cells deposit their products on the surface of the larval tunic in late embryogenesis. In these species, the test cells are lost at hatching. In other species, the test cell vacuoles contain acidic glycosaminoglycans, but no ornaments. In these species, the test cells attach to the larval tunic and probably secrete acidic glycosaminoglycans. We deprived the embryos of seven species of ascidians of their test cells and vitelline coats during midembryogenesis. After completing their development, the larvae of both kinds of species were hydrophobic. They easily become trapped on the surface of sea water in cultures. Normal larvae (controls), bearing test cell secretions, are hydrophilic and never become trapped. We infer that negatively charged secretions of the test cells make normal larvae hydrophilic. Some molgulids with direct development have no test cells, no fins and no swimming larva. We reason that the test cells of these species may have been lost during evolution because they no longer had an important role in preventing hydrophobicity.