The microscopic and submicroscopic structure of the tunic of two ascidians, Boltenia and Molgula

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Abstract
The structure of the tunic of two ascidians, Boltenia ovifera and Molgula griffithisii, was investigated by optical microscopy, X-ray diffraction, and electron microscopy.The outer surface of the tunic of Molgula consisted of a single layer of nucleated cells, with numerous protrusions 0.5–1.0 mm long. Inside this layer of cells, the cellulose microfibrils, which are a major component of the tunic, were randomly oriented within the plane of the tunic. No inner cellular layer was observable.The tunic of Boltenia had numerous, pointed spines on the outer surface but no cellular organization within the tunic was apparent. The spines were composed of parallel microfibrils (perhaps cellulose) oriented axially. The microfibrils of the spines were more densely packed and well oriented at the rim of the spines. In general, the arrangement of the cellulose microfibrils within the tunic was random within the plane of the tunic although there were some areas displaying optical birefringence and orientation of microfibrils.The observations are consistent with present knowledge of the mechanism of formation of cellulose microfibrils in green plants and bacteria but throw little light on the mechanism of morphogenesis of the tunics themselves.

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