DEEP-SEA LARVAL EYES 405 



the cheek, just behind the eye, which is reminiscent of the conditions in 

 Photoblepharon and Anomalops (Fig. 134). 



There are other stalk-eyed deep-sea fish larvae, notably those of 

 Bdtbylagus, Eustomias, and certain myctophids; but none can compare 

 with 'Stylophthalmus'. 



In the literature of comparative ophthalmology, one deep-sea fish, 

 *Scopelu5 caninianus* (= Myctophum punctatum) is erroneously credited 

 with having 'telescopic' eyes as a larva, the eyes becoming normal in the 

 adult. The eyes are indeed elongated in this and in some other species 

 of Myctophum; but the elongation is not axial, but vertical — the vertical 

 diameter of the eye greatly exceeding the horizontal and the axial diam- 

 eters, which about equal each other. The eyeball is often pointed 

 inferiorly, but it always rounds up during metamorphosis. In these 

 Myctophum species the adult eye is aimed sidewise; but the larval eye 

 for a time looks directly forward, and thus deserves the adjective *prae- 

 scopic' equally with Argyropelecus (Fig. 137a), to which this term has 

 been applied. 



The ecological significance of praescopic and stalked larval eyes is 

 quite unknown. At first thought, one might suppose that they afforded 

 superior perception of distance through enlargement of the binocular 

 field or by increasing the length of the inter-ocular base of the range- 

 finding triangle. But these larvae are only a few millimeters in length, and 

 their ocular frontality and relatively large inter-ocular distances are very 

 temporary in the life-cycle, and may have no meaning for binocular 

 vision — or at least, not the meaning they would have in sizable animals. 

 Even among large fishes, there are some which only seem to have taken 

 special pains regarding distance-perception. The hammerhead sharks, 

 for example, have their eyes very far apart, at the ends of the 'hammers'; 

 but they gaze only laterally, and apparently their monocular visual fields 

 are overlapped but slightly if at all. 



The Common Eel — A really amazing case is that of the common eel, 

 Anguilla bostoniensis. The biological world was startled when the fairy- 

 tale life history of this drab fish was finally worked out a few years ago. 

 One of the most fantastic things about the eel is the cycle of change 

 through which its eyes pass: 



As we see eels during the long vegetative existence of the females in 

 our inland ponds and streams, their eyes are small, hypermetropic, cov- 

 ered by a spectacle, and apparently semi-degenerate like those of 



