The pinealocytes of fishes, amphibians, reptiles, birds and mammals have been compared with cerebrospinal fluid (CSF) contacting neurons. We found that the intraventricular dendrite terminal of the latter resembles the pinealocytic inner segment and that the atypical cilium (9x2+0 tubules) of the CSF contacting neurons is analogous with the outer segment of the pinealocytes, even though the outer segment bears photoreceptor lamellae in lower vertebrates. Regular, but small-sized photoreceptor outer segments were also found on pinealocytes of the chicken. In mammals, too, primitive outer segments are present in the form of 9x2+0 cilia similar to those of CSF contacting dendritic terminals.
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