Development of the larval tunic in a compound ascidian: morphogenetic events in the embryos of Distaplia occidentalis

Abstract

The larval tunic of Distaplia occidentalis is a complex investment consisting of extracellular filaments and an amorphous ground substance. The principal elements of the tunic are two thin cuticles and two subcuticular compartments. Compacted filaments and small amounts of ground substance characterize the cuticles. Dispersed filaments and large amounts of ground substance distinguish the compartments. The cells of the embryonic epidermis apparently secrete both the filaments and the ground substance. The elements of the larval tunic differentiate sequentially over a period of approximately 2 weeks. The outer cuticle appears 3–4 days after neurulation and it gradually lifts from the surface of the epidermis as the outer compartment forms. Tapered folds of the outer cuticle and extensions of the outer compartment produce the dorsal, ventral, and posterior fins of the larva. Test cells, occupying the perivitelline space around the embryo, release multigranular "ornaments" that adhere to the external surface of the outer cuticle. The inner cuticle and the inner compartment arise during the 4 days prior to hatching. The inner compartment over the truncal epidermis expands significantly to accommodate cells which emigrate from the body cavity (hemocoel).