454 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



fixed in the head skin while the cornea slides under it with the rotational 

 movements of the eyeball. 



A half-way stage in the production of a secondary spectacle seems to 

 be exhibited by certain of the Cottidae — Ascelichthys rhodorus for ex- 

 ample. Here, the usual circumocular sulcus (Fig. 151b, s) has been elim- 

 inated. All that remains to represent the formerly infolded conjunctiva 

 is a narrow zone of puckered skin surrounding the cornea and merging 

 with the head skin at the rim of the orbit. The circular, concentric pleats 

 in this skin afford the leeway required when the eyeball turns in the orbit. 

 If the surface layers of the cornea continuous with the skin should split 

 off, the pleated zone could then shrink in area and obliterate its pleats. 

 Ascelichthys would then have a typical secondary spectacle. 



Fig, 152 — Types of spectacles in teleost fishes. After Hein. 



a, secondary spectacle of Anguilla angiiilla. ep- epithelium of spectacle (= original corneal 

 epithelium); sc- separated portion of cornea, forming mass of spertacle; st- strands of 

 delicate conneaive tissue, which do not interfere with the movement of the eye beneath the 

 spectacle; pc- primitive cornea, which remains continuous with sclera, b, tertiary spectacle 

 and eye (collapsed) of Engraulis sp. c- cornea; e- epithelial lining of: ics- intraconjunctival 

 space; s- sclera; sk.- skin of head; sp- spectacle. 



Tertiary Spectacles in Reptiles — The tertiary spectacle is a type 

 with which most of us are familiar, for we have all noted the glassy stare 

 of the reputedly lidless serpent. The snake does have lids; but they have 

 been closed for all time, and converted into the hard, horny, dry trans- 

 parent, insensitive eye scale of the herpetologists (Fig. 15 Id). There was 

 long a debate as to whether this spectacle represented the upper and lower 

 lids, the lower alone, or the nictitating membrane or third eyelid inherited 

 from the lizards. Recent embryological work on the European grass 

 snake and on one of the rattlesnakes has shown that after the formation 



