TERTIARY SPECTACLES IN FISHES 459 



section D) that the 'first snake' must indeed have lived underground, 

 and for so long a time that the eyes degenerated very gravely. When the 

 snakes later became a diversified, largely above-ground and diurnal 

 group, they all had perforce to keep their spectacles whether they had 

 any particular need for them or not. 



As long as its surface is capable of being renewed, a tertiary spectacle 

 is clearly more desirable for any land animal than is a pair of movable 

 lids. We ourselves would be better off with one, and it is perhaps unfor- 

 tunate that none of our direct ancestors ever had habits which made a 

 tertiary spectacle mandatory. We are forever getting 'something in the 

 eye' — and when this happens, we are able to deal with the situation only 

 because we have intelligent fingers. A wild animal, in the same predica- 

 ment, may claw its eye until it is injured beyond repair. 



But, it would seem that nature can be persuaded to seal the terrestrial 

 vertebrate's lids, and make them into a spectacle, only where the eyeball 

 is imperilled far more seriously than it ever is in the vast majority of 

 lidded vertebrates. 



Tertiary Spectacles in Fishes — Spectacles assignable to this category 

 occur in a few teleost fishes, where they differ from those of snakes and 

 lizards in almost every particular excepting the most fundamental one 

 of diagrammatic morphology. 



In contradistinction to the secondary spectacle, so widespread among 

 bottom forms, the tertiary type occurs only in a few pelagic fishes which 

 are mostly close relatives of fishes in which vertical lids (p. 383) are con- 

 spicuously developed. Ichthyologists have apparently assumed that all 

 intact coverings over piscine eyeballs represent closed, fused, vertical lids. 

 Some taxonomic descriptions of fishes forthrightly call the spectacle, 

 whether primary, secondary, or tertiary, an 'imperforate adipose lid'. 

 Clearly, there is need for a more careful identification of types, as other- 

 wise some gravely erroneous taxonomic conclusions might be drawn. 



Externally, in fishes, there is no way to distinguish the three types of 

 covering. Histologically, the primary and secondary types are discrimin- 

 able only on the basis of the absence or presence of an intermediate 

 system of delicate strands connecting the spectacle with the functional 

 cornea (Fig. 152a). Where these strands are particularly strong, it may 

 be possible to see them grossly upon cautiously reflecting the circumcised 

 spectacle from the cornea. Some have claimed to find such connections 

 between the two structures even in lampreys. However strong or weak 

 these connective-tissue strands may be, they never prevent the cornea 



