This chart compares development of a generic gastropod larva (fig. I & II) illustrating the velum and formation of the mantle cavity and operculum. The third image is the larva of a sea butterfly, the classification of which remains confused to this day. Sea butterflies were thought to be similar to sea angels because of their flapping appendages developed from the food however these two groups are now thought to be quite different. Note the position of the forming mantle cavity relative to the operculum and velum between I & III, the sea butterfly has not undergone torsion.