A direct visualisation of helicoidal architecture in Carcinus maenas and Halocynthia papillosa by scanning electron microscopy.

The test of the Tunicate Halocynthia papillosa is shown to have a laminate organization. Three properties of a helicoidal model can be directly observed: (1) Fibres in each lamina run parallel. (2) Fibres in adjacent laminae run at an angle. (3) In oblique section, discontinuous parabolic arcs made up of straight lengths of fibre from succeeding laminae are observed. These properties are confirmed in the cuticle of Carcinus maenas and the relationship of the pore canals to the laminar organization is discussed. It is shown by matching light and electron micrographs that the 'macrofibres' of recent workers are displaced pore canal tubules.