ACCOMMODATION IN FISHES 263 



define the latter rather arbitrarily as 'the portion of the anterior uvea 

 which is visible through the cornea', in order to distinguish the remain- 

 der as a ciliary body. 



The ciliary muscle fibers run from the inner surface of the rim of the 

 cornea to the outer surface of the chorioid at or near the ora terminalis, 

 and because of this disposition were long called a 'tensor chorioideae' 

 muscle, and were believed to tauten the chorioid around the vitreous to 

 maintain turgidity and an unvarying optical situation during the move- 

 ments of the lens. The chorioid is too firmly plastered onto the sclera 

 anteriorly to make the need of such an action plausible, however, and at 

 present we are helpless to explain the teleostean ciliary muscle as any- 

 thing but a phylogenetically precocious, 'orimentary' or pre-adaptive 

 structure, of unknown but minor importance, which very conveniently 

 hung on until the reptiles found an important job for it. 



By and large, teleosts are more or less myopic — up to as much as 15 

 diopters, the highly abnormal telescope gold-fish even more so. This is 

 to be expected, since approaching the lens to the retina would only put 

 their eyes out of focus for any and all distances if they were not myopic 

 to start with. Their eyes are thus set for close work with a minimum of 

 effort, and they need to exert muscular force only when attending to 

 distant objects — and 'distant', for the average fish in the average natural 

 body of water, means only up to fifty feet at most. Beyond this distance 

 underwater vision — anything more than light-sense — is practically nil. 

 Many mud-grubbing, small-eyed fishes are hypermetropic, indicating a 

 loss of importance of vision to them, for which we will see an exact 

 analogy among the mammals (v. i.) . 



Other Fishes — Of these we can say little. Nothing is known concern- 

 ing accommodation in the living cladistians, Polypterus and Calamoich- 

 thys. Nothing whatever is known as yet concerning the eye of the newest 

 'living fossil', the crossopterygian Latimeria cbalumnae. 



Dipnoans appear to have no accommodation. In the small-eyed forms, 

 at least, there is no ciliary body, no zonule, no lens-muscle. In Lepido- 

 siren and Protopterus this is comprehensible, for the whole eye, and par- 

 ticularly the retina, is so very crudely built as to make accommodation 

 a useless refinement. The relatively large eye of Neoceratodus deserves 

 further study. This lungfish does not aestivate in mud, and spends much 

 of its time at the surface of the water (see Fig. 61a, p. 137), where its 

 eyes should be quite useful. 



