Fig. 9

Proposed homology of the fundus-forming cell of Stenostomum and rhabdomeric photoreceptors of Rhabditophora. Each of the cells can be divided into four corresponding regions showing positional and ultrastructural similarities: the apical microvillar region (in red) is equipped with prominent microvilli and harbors numerous vesicles and mitochondria, the microvillar region is connected by the neck region (in green) to the nuclear region (in blue), that is positioned outside of the receptive organ, finally, the cell connects to the brain neuropile by axon (in magenta). Drawings of rhabdomeric photoreceptors after Carpenter et al. (1974) [55] (for triclads) and Bedini and Lanfranchi 1998 [54] (for rhabdocoels). Abbreviations: ax axon, cps cytoplasmic processes, mt mitochondria, mv microvilli, nu nucleus, ob onion bodies, ve vesicles