REVIEW OF LARVAL AND POSTLARVAL EYE ULTRASTRUCTURE IN THE LAMPREY (CYCLOSTOMATA) WITH SPECIAL EMPHASIS ON GEOTRIA-AUSTRALIS (GRAY)

The literature dealing with the lateral eye in lampreys is briefly rev iewed here. While there appears to be no longer much doubt that the sh ort and long photoreceptor cells in the lamprey eye correspond to rods and cones, questions of dark/light adaptational changes, the nature o f visual pigments, and the roles of retinal serotonin and melatonin ne ed to be re-addressed. Eyes of the larval and postlarval (''macrophtha lmia'') stages of the lamprey Geotria australis were examined by elect ron microscopy and it was found that the larval retina is largely undi fferentiated except for a small central zone surrounding the optic ner ve head. The retina of the postlarval stage is fully differentiated an d the photoreceptor outer segments undergo renewal, which appears to i nvolve the retinal pigment epithelium (RPE). The distribution of larva l RPE and choroidal pigments, postlarval ganglion cells, and the orien tation of scleral collagen are unusual for vertebrates. No obvious pos itional or size differences of any retinal cell type were apparent bet ween day- and night-fixed specimens. (C) 1996 Wiley-Liss, Inc.