384 ADAPTATIONS TO MEDIA AND SUBSTRATES 



The salmonoids (salmons, trouts, whitefishes) present a condition 

 which differs considerably from both the two-iids-overlapped and contin- 

 uous-circular-fold extremes, and from any situation intermediate between 

 them. The salmonoid complex (Fig. 132a) consists of a narrow, crescentic 

 posterior lid running around two-thirds of the circumference of the eye 

 (and comparable with the posterior lid of a herring or a mackerel) 

 together with a broad, roughly triangular, anterior fold. The latter is 

 depressed below the surface of the head, for it is developed not from the 

 extreme margin of the circumocular sulcus, but as a separate conjunctival 

 fold arising from beneath that margin, on the anterior side of the mem- 

 branous orbit. 



It is hard to say whether this arrangement has been derived from one 

 like that of the clupeoids, or is quite independent. Ecologically, it prob- 

 ably has a special significance. The eye is not actually as well stream- 

 lined as it would be if the anterior sulcal margin were to recede smoothly 

 into the head surface, thereby creating something more like the arrange- 

 ment in Hiodon. In the salmonoids, the bony orbit is incomplete anteri- 

 orly, and it may well be that they have taken the opportunity to draw 

 the anterior sulcal margin well forward, primarily to permit of more 

 straightforward vision and a wider binocular visual field during the pur- 

 suit of prey. The broad, stiff, anterior lid-fold of the salmonoids, which 

 has been called a 'false nictitating membrane' (Fig. 132a, fn), can thus 

 be thought of as having been left behind by the forward-migrating sulcal 

 margin (to prevent the opening up of a gap between the latter and the 

 cornea), rather than as having grown actively, posteriorly, toward the 

 center of the cornea as the anterior lids of the clupeoids and scombroids 

 have certainly done. 



Bottom Fishes — A host of coastal fishes, both elasmobranchs and tele- 

 osts, have chosen to live on the bottom. By thus putting their backs 

 against a wall and living at the center of a hemisphere of space rather 

 than a sphere, they have halved the job of watching out for enemies and 

 prey. At the same time they are close as can be to a retreat or a cam- 

 ouflage — in crevices or burrows, or in the sand or mud with which they 

 can cover themselves. Living as they do in such intimate contact with 

 their chief food supply, the other members of the 'benthos' or bottom 

 fauna, many crevice- and mud-dwelling fishes have found vision of little 

 use, and have allowed their eyes to become small or degenerate — or even 

 to dwindle to tiny, blind remnants under an opaque skin (Fig. 133b, 

 p. 387). 



