Fig. 1

A. filiformis non-regenerating arm skeletal elements and ultrastructure of cell types. A 3D schematic representation of a non-regenerating arm showing the main anatomical structures colour coded as in legend on the left. B SEM of the skeletal elements present in a single metameric unit and displayed in their corresponding anatomical position. AP, aboral arm plate; LP, lateral arm plate; OP, oral arm plate; S, spine; V, vertebra. C SEM micrograph of a cell in the stroma of the skeleton of the vertebra: arrowhead indicates the nucleus, white arrow points at fibrils with morphology typical of collagen, magnified in C’. D TEM micrograph of a sclerocyte in the vertebra: white arrows indicate fibrils in cross section likely of collagen, magnified in D’. E TEM micrograph of a presumptive pigment cell in the oral arm plate with an evident nucleolus and spindle-shaped electron-dense structures typical of pigment granules, magnified in E’. F TEM micrograph of a phagocyte in the aboral arm plate characterized by the presence of phagosome, magnified in F’. G TEM micrograph of a granulocyte present in the vertebra: red arrowheads highlight fibrils, present in both cross and longitudinal sections; arrows point at large electron-dense granules of different shapes and sizes, magnified in G’. H TEM micrograph of a nerve cell in the aboral arm plate characterized by a nerve process magnified in H’; white arrow indicates the small electron-dense roundish vesicles generally containing neurotransmitters or other neuro-signalling molecules. Scale bars = 2 μm. In all images: asterisks indicate the presence of biomineralized skeletal tissue (stereom calcareous elements or trabeculae) now empty, i.e. electron-transparent due to fixation and decalcification processes; white arrowhead shows the nucleus with hetero- and euchromatin; and red dotted box indicates magnified part of the figure