Formation of the test in the swimming larva of Ciona intestinalis: an ultrastructural study.

The test of the Ciona intestinalis larva, before hatching, presents a single outer cuticular layer, C1. Ultrastructural observations of swimming larvae, kept in culture at 18 degrees C, show that the test is enriched by a second inner cuticular layer C2. The C2 layer begins to form in the larva immediately after hatching (about 1 h) and is a process caused by the activity of the cells of the ectodermal layer. During this phase of larval development these cells assume the typical ultrastructure of cells with secretory activity, presenting a well developed r.e.r. and a Golgi in the form of dictyosomes in active synthesis phase in the apical zone of their cytoplasm. The formation of C2 layer is complete in the larva 3 h after hatching and in this period it can be seen parallel to, and a short distance from, the plasmalemma of the ectodermal cells. In the cephalenteron region the C2 layer progressively rises towards the C1 layer and, in the larva 8 h after hatching, these two layers are very close together, about 1000 A apart. In contrast, the C2 layer in the tail remains close to the plasmalemma of the ectodermal cells and can only be detected by ultrastructural observations.