214 BONY FISHES vn. 14 



retina. However, this has recently been disputed by Verrier, who 

 denies that the campanula is muscular and believes the eye to be 

 hypermetropic at rest and accommodated, if at all, by fibres of the 

 ciliary body, as in other vertebrates. It may be that the fixed focus is 

 already sufficiently deep and the campanula perhaps serves mainly 

 to steady the lens. 



It is unwise, however, to generalize about teleostean eyes, for they 

 are very varied. Whereas the trout, like most, has a round pupil, 

 which varies little if at all in size, other fishes, whose eyes are more 

 exposed to light from above, have a more mobile iris. In flat-fishes 

 and the angler-fishes, such as Lophius, and the Mediterranean 

 Uranoscopus, the star gazer, the iris has an 'operculum' and is very 

 muscular; its movements are controlled by nerves and not, as in 

 selachians, by the direct effect of light. The sympathetic system sends 

 branches into the head in these animals (Fig. 138) and its fibres cause 

 contraction of the sphincter of the iris, whereas fibres in the oculo- 

 motor nerve cause contraction of the dilatator, the opposite arrange- 

 ment to that in mammals. In the eel the pupil is also capable of wide 

 changes of diameter, but here the control is mainly by the direct 

 response of the circular sphincter iridis muscle to light incident upon 

 it. The pupil of the isolated eye of an eel closes when illuminated and 

 reopens again in darkness (Fig. 131). Presumably because of its lack 

 of nervous control this iris is not affected by many of the usual 

 'autonomic' drugs. For instance, closure will occur in the presence of 

 atropine and the dark-adapted pupil remains unchanged when placed 

 in a solution as strong as 1 per cent, pilocarpine, but then closes 

 immediately on illumination (Fig. 132). The isolated pupil of Urano- 

 scopus, however, closes when pilocarpine is applied (Fig. 133), and 

 in this case the sphincter muscle is innervated by sympathetic nerve- 

 fibres. Adrenaline also causes the sphincter to contract and acetyl 

 choline in moderate concentrations causes dilatation 



The eyes may be small or absent in fishes living in caves, muddy 

 waters, or the deep sea. In this last habitat, however, many have ex- 

 ceptionally large eyes, with, apparently, high acuity as well as sensi- 

 tivity. They may be elongated ('telescopic') and with large binocular 

 fields and a fovea of 'rods'. In Bathylagus the rods reach a density 

 of 800,000 mm 2 and are arranged in six superimposed layers, which 

 presumably come into action successively as an object approaches. 

 The cells of the deeper layers are less closely packed. 



The tropical fish Anableps lives with the head half out of water and 

 the eyes are adapted for use in both media. The upper part of the 



