402 ADAPTATIONS TO MEDIA AND SUBSTRATES 



accommodation — the lens is often as big, in proportion to the head, as is 

 the entire eye of such a fish, even, as ^enion (y.s.). 



In such genera as Gigantura and Winteria, where the definitive tubular 

 eye aims forward (Fig. 138b), similar intra-ocular rearrangements are the 

 basis of the change in external form. The lens migrates nasally, of course, 

 rather than dorsally. In Bathytroctes and Platytroctes the eyes are appar- 

 ently in a half-way stage in evolution toward an eventual forward-aimed, 

 tubular organ. Bathytroctes is almost unique in having a fovea in its 

 pure-rod retina (see p. 190). 



The utility of the upward aim of so many tubular eyes is not entirely 

 clear, but it may be associated with the orientation of light-producing 

 organs. Contrary to common supposition, the luminous organs of nearly 

 all aquatic animals aim their light downward, not sidewise. This is true 

 of the many luminous cephalopods mentioned above, and also of the 

 fishes, both elasmobranchs and teleosts. Where the light comes from a 

 broad area of skin, as in the luminous shark Etmopterus, this area is 

 located on the underside. Where there are discrete photophores built like 

 eyes, with lenses and reflectors, these aim downward — or, if located on 

 the sides, they are so arranged that 80% of the light goes downward, not 

 horizontally. There may be a few photophores on the back, but Hubbs 

 has noted that these are always tiny and often appear to be degenerate 

 in structure. In the few instances in which photophores shine frankly 

 horizontally, they differ in numbers and arrangement in the two sexes, 

 and here they are obviously serving primarily as sexual recognition-marks. 



The downward aim of the light seems reasonable enough in a demersal 

 species; but, it is just as characteristic of the many bathypelagic fishes 

 which live by day at 200 meters or so and come to the surface at night — 

 the myctophids or lantern-fishes, the sternoptychids, Astronesthes, Cyclo- 

 thone, etc. In such vertically migratory forms, most of which school in 

 large numbers, one might expect the light to be aimed sidewise or even 

 upward. But whatever the significance of the orientation of photophores 

 may be, it does seem likely that the upward aim of tubular eyes is in 

 sympathy therewith. The deep-sea fish is not much concerned with trying 

 to see objects illuminated by his own photophores — rather, he sees other 

 organisms by means of their photogenic organs, and his own serve chiefly 

 as a lure for prey and as an identification-tag for others of his own kind. 



The parallelism of the optic axes of all deep-sea tubular eyes, (whether 

 these are aimed upward, or forward) , in itself poses a special question. 

 Why should forms with such unsharp vision have such extreme binocu- 



