Ultrastructural survey of tunic morphogenesis in the larval and young adult ascidian Ascidiella aspersa (Tunicata, Ascidiacea).

In Ascidiella aspersa, tunic development begins during the tail-bud stage. In the trunk, the larval cuticle and three tunic layers arise consecutively. In the tail, just the larval cuticle and one tunic layer are formed. Shortly before metamorphosis, a new very thin layer arises limiting the tail tunic interiorly. The tunic is composed of granular and fibrillar material embedded in an electron-transparent ground substance. In the course of metamorphosis, the tail is retracted. The larval cuticle and the first tunic layer are discharged. The young ascidian is just surrounded by a cuticle and one tunic layer.