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Chemical, histochemical and electron microscopic studies on the fibrous polysaccharide component of the test of Ciona intestinalis L. were made. The results showed cellulose-like fibres to be present in the cuticle layer and in the ground substance. Bundles of fibres varying in diameter from 80 to 600 nm interweave to form a conspicuous network. The bundle appears to be formed of flat fibres, 10–13 nm in width, while the fibre is composed of fibrils 3.5–5 nm; the rectangular cross section of fibres can be supposed. The probable role of this polysaccharide is discussed.

Ciona intestinalis L. tunic architecture and cell distribution were investigated with the electron microscope. The observations showed that the ascidian covering is formed by a thin outer cuticle, a subcuticle of variable width and a large single layer of ground substance. “Large granule”, morula, phagocyte and granulocyte are the cellular types encountered; they appear mainly in highly vacuolated states and are distributed throughout the whole tunic. The “large granule” cells, however, are mainly seen in the cuticle layer and the morula cells appear mostly in the outer zone of the ground substance. The role of these cells in tunic construction, repair and regeneration as well as their scavenging function are discussed.