90 FREDERICK TILNEY AND LUTHER F. WARREN 



What has been termed a parietal cornea of the pineal eye con- 

 sists of a layer of almost fiberless tissue of considerable thickness 

 between the dorsal surface of the pial capsule and the inner 

 surface of the bony depression in the skull. The epidermis 

 immediately above this so-called cornea is quite without pig- 

 ment, forming a small, circular area in the frontal region of the 

 head situated almost immediately in the midsagittal line. This 

 area was recognized long before its significance was understood 

 and was described by Whitwell 421 in 1888, by Ahlborn 2 in 1883, 

 and by Gage 135 in 1893. Gaskell 145 in 1890 erroneously likened a 

 cranial thickening above the pineal organ in Ammoccetes to the 

 cuticular lens of Arthropods. Studnicka 384 in 1893 found that 

 the cornea is discernible in the 25 mm. Ammoccetes. Gaskell, 145 

 in his discussion of the origin of vertebrates from a crustacean- 

 like ancestor, makes the statement that in Ammoaztes there 

 are two pineal eyes, one, dorsally placed, much larger and in- 

 tensely white in color, lies in front of the right habenular ganglion. 

 The other, ventrally placed, is an insignificant structure. The 

 first is similar to the crustacean parietal eye in its pigmentary 

 character. The second is similar to this eye in Crustacea because 

 of the termination of the nerve endings with the attached rhab- 

 dites. According to Gaskell, the type of eye is clearly arthro- 

 podic. The arrangement of the nerve endings, the shape of the 

 internal cavity, and the position and simplicity of the attached 

 rhabdites all point to larval characteristics and, therefore, to an 

 ancient type. The anterior wall is not a lens. Gaskell believes 

 the lens is cuticular in character and, if so, this is all the more 

 reason for believing that the pineal eye is definitely arthropod in 

 type. 



Much emphasis has been laid upon the occurrence in cyclo- 

 stomes of these two structures which have so many characteristics 

 suggestive of visual function. The statement has been made 

 that this is competent evidence upon which to establish the 

 claim that in vertebrates the parietal or third eye was primi- 

 tively paired. It is to be noted, however, that in no other class 

 of vertebrates does the duality of the parietal visual apparatus, 

 if such indeed it may be considered, attain such a high degree of 



