A light- and electron-microscopic study of the pineal complex of the ammocoete larva of the southern lamprey Geotria australis.

The pineal complex of larval Geotria australis lampreys has been examined by light- and electron-microscopy. The complex consists of a pineal organ and a smaller parapineal organ, the former being situated dorsal to the latter. It is concluded that the pineal organ is a functioning photoreceptor, with neural and endocrine output. The parapineal appears to be a more basic neuroendocrine organ without photosensory capability. Day-night comparisons of the pineal complex show no obvious differences in ultrastructure; this could be related to the chronobiological role of the complex as a constant monitor of ambient light levels, rather than a mere visual receptor that changes its sensitivity at night.