Convergent evolution of the sensory pits in and within flatworms.

Since there is no morphological or molecular correspondence between the sensory pits of Stenostomum and the cerebral organs of nemerteans, we reject their homology. Interestingly, the major cell type contributing to the sensory pits of stenostomids shows ultrastructural similarities to the rhabdomeric photoreceptors of other flatworms and expresses ortholog of the gene pax4/6, the pan-bilaterian master regulator of eye development. We suggest that the sensory pits of stenostomids might have evolved from the ancestral rhabdomeric photoreceptors that lost their photosensitivity and evolved secondary function. The mapping of head sensory structures on plathelminth phylogeny indicates that sensory pit-like organs evolved many times independently in flatworms.