Fig. 7

Sensory cells under different stainings exemplified for photoreceptors. A-DLineus viridis (Nemertea), anterior end. A Bright-field microscopy. Note that shading pigment marks the photoreceptors (arrows). B Azan staining of 5 μm thick section, bright-field. Receptor cells (r) are partly surrounded by shading pigment cells (p). Asterisk marks receptive region that can easily be identified by its homogenous staining and its apical position; neither single receptor cells nor the structure of the receptive region can be identified. Arrow marks direction of incident light. C Transmission electron microscopy, sagittal section; eye outline marked by dashed line. Shading pigment cells contain numerous vesicles with electron grey to electron dark content. D Detail of the receptor cells (rc) that consists of a dendritic process with numerous apical microvilli (mv) and a single cilium (arrow). Both, cilium and microvilli are constitutive for a certain class of photoreceptor cells. Note differing electron density of the vesicle content shading pigment cells (pc). Arrow heads mark adhaerens junctions indicative for epithelial sensory cells