Visual behavior, eye and retina of the parasitic fish Carapus mourlani.

1. This is the first transmission electron microscope description of the retina of a parasitic fish. 2. The retina is well supplied with capillaries, some of which occur on the inner surface of the retina. An unusual and possibly primitive type of capillary junction is described. 3. The outer segments of the retina of Carapus mourlani, which measure approximately 2 micron in diameter, are of one type only. A certain degree of banking was observed. Outer segments are not isolated from each other by screening pigments; they are, however, surrounded by about 15 calycal processes. 4. Typical spheroids or conoids were not seen in the outer plexiform layer. The inner nuclear layer consists of only one or two layers of nuclei. The inner plexiform layer is unusually wide and some amacrine synapses can be identified. There are few ganglion cells and the ratio of nuclei in the outer nuclear layer to those of the inner nuclear layer to ganglion cells is approximately 100:10:1. 5. The eye of C. mourlani combines features of degenerated photoreceptors, characteristic of cave-organisms, with adaptations which are commonly found in nocturnal and deep-sea forms. 6. The visual behavior of the animal indicates that the eye is fully functional in spite of its corneal keratosis and small number of optic nerve fibers.