Ultrastructure of Terebratalia transversa larval eyes. (A, B) Brightfield microscopy of a Terebratalia transversa larva, with red eye spots visible in the apical lobe (black arrows). (A) Dorsal view. (B) Lateral view. (C-F) Ultrastructure of eyes in the larva of Terebratalia. (C) Longitudinal section through whole larva with eyes (black arrows) on either side of the apical lobe. (D) Two neighboring eyes with lenses (ls) in the lens cells and pigment granules (pg) in the shading pigment cells, separated by two epidermal cells. Yolk granules are present in both cells (y). (E) Detail of a pigment cell showing the enlarged membrane (asterisk) of its sensory cilium (arrow). (F) Detail of the enlarged ciliary membranes of both the lens cell (lc; black arrow) and the pigment cell (pc; black arrowhead) that fill the optical cavity. (G) Receptive cilia of both photoreceptor cells have a typical 9 × 2 + 2 microtubule pattern, exemplified by a cross section of the lens cell cilium (black arrow). (H) Reconstruction of larval eye axon tracts from serial sections. Axons from the lens cell and pigment cell extend to the apical ganglion (green), which overlays the mesoderm (pink) and endoderm (blue). (I) Reconstruction of a larval eye of Terebratalia from serial sections, consisting of a lens cell and a shading pigment cell. Notice the two enlarged sensory cilia of both cells (black arrows) and the proximal axons.