There is a wide variety of different sensory organs, but the basic principles of their function are similar. This chapter summarizes the known sensory structures, and ciliary receptors are briefly discussed. The two sensory structures discussed in detail are static sense organs and photoreceptors. Static sensory organs occur as statocysts in a variety of animals. They differ extremely in shape, composition, and location, making their multiple parallel evolution quite likely. Photoreceptors also occur in abundance and in different forms. Following a review of photoreceptors among animal taxa, it is concluded that the molecular photosensitive system is quite old and that it is, for example, likely that several opsins were present in the bilaterian ancestor. The particular architecture of the photoreceptor (‘eye’) evolved in parallel many times over.
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