DEEP-SEA FISHES 403 



larity? Where there is nothing to see but a few dots of light once in a 

 while, what price such a provision for refined space-perception? Probably, 

 the binocularity is desirable chiefly because of the impossibility of accom- 

 modation and convergence in tubular-eyed fishes, coupled with the fact 

 that the usual monocular cues to distance (p. 314) are lacking in the vel- 

 vety blackness of the depths. And, probably, binocularity would be just 

 as useful in the large, normally-shaped eyes of other deep-sea fishes — 

 but in them, it could not be so easily arranged for. In the creation of the 

 tubular form, there is opportunity to swing the visual axis through an 

 exceptionally wide angle. Such forms as Gigantura have simply carried 

 to a great extreme the same nasad asymmetry which many other animals 

 have employed as a device for widening the binocular field (see p. 300). 



Fig. 138 — Deep-sea teleosts with tubular eyes. After Brauer. 



a, with eyes aimed upward (Opisthoproctus soleatus). Redrawn, b, with eyes aimed for- 

 ward (head of 11.8cm. Gigantura chuni). 



If the reader will imagine trying to estimate the distance of a faint 

 dot of light in a darkroom, with one eye closed, he will appreciate the 

 value of having bearings on such a stimulus from two angles at once. 

 The deep-sea fish never has much more to look at than the photophores 

 of his scanty neighbors. Monocularly, he would be about as helpless to 

 localize them accurately, as we are to judge the distance of the stars. 



Deep-Sea Larval Eyes — Ordinarily the eyes of larval deep-sea fishes 

 are normal in structure — for larval teleost eyes — and take on any peculiar 

 conformations, such as the tubular form, during metamorphosis and 

 adolescence. Here, ontogeny is repetitive of phylogeny. In a few in- 

 stances, however, this course of events is reversed, and a bizarre larval 

 eye becomes an orthodox adult organ. 



Most outstanding is the case of 'Stylophthalmus paradoxus', a larva 

 first described by Brauer in 1902. Not until 1934 was it established, by 



