Early stages of test formation in larva of Ascidia malaca (Tunicata, Ascidiacea): ultrastructural and cytochemical investigations.

The swimming larvae of ascidians are entirely covered by a hyalin coat called tunic, or test. This covering consists of two cuticular layers, C1 and C2, which surround an inner compartment composed of an amorphous hyalin matrix with numerous fibrils dispersed inside. Data from the literature agree on the key role played by the cells of the larval ectodermic layer in the synthesis and secretion of larval test components.