Retinofugal and retinopetal projections in the Pacific hagfish, Eptatretus stouti (Myxinoidea).

The retinofugal and retinopetal connections of the Pacific hagfish were determined using the fluorescent compound DiI as an in vitro tracer. The retinofugal connections were found to be bilateral and to reach preoptic, thalamic and pretectal, as well as tectal, targets. No distinct basal optic root was found. A retinopetal projection in hagfishes is described here for the first time. It originates from two cells groups in the rostral mesencephalic tegmentum. These retinal connections in hagfishes are compared to the organization of the visual system in the second Recent group of jawless fishes, the lampreys. Both visual systems are similar in the number and location of the targets of retinal fibers. Furthermore, there is a striking similarity in the retinopetal systems. Major differences are found in the organization of the optic tract. These observations allow the identification of some cell groups not previously recognized in myxinoid brains or interpreted differently. Finally, the findings are discussed in relation to visually guided behavior in hagfishes.