No. 8. — The Parietal Eye in some Lizards from the Western United 



States. By W. E. RiTTEK.i 



With a single though notable exception, the numerous authors who 

 have written on the parietal organ in vertebrates since the papers of 

 de Graaf ('86^ and '86^) and Spencer ('86 and '87) appeared, have 

 agreed that the structure is, or at least was in ancestral vertebrates, 

 an eye. This belief is based entirely on the structure of the organ, no 

 physiological experiments or observations on the habits of the animals 

 possessing it having yet been produced in proof of its function. 



Leydig ('89) alone, in a recent preliminary paper on the subject, has 

 denied its optical nature, and has assigned to it an entirely diti'erent 

 function : though in a second preliminary, still more recent ('90), he 

 expresses his denial with considerably less confidence. He rejects the 

 eye hypothesis, however, on the same grounds that have led others 

 to adopt it ; namely, on the grounds of its structure, and especially 

 of its relation to the brain. 



He believes that what is generally held to be an optic nerve is in fact 

 merely a string of connective tissue. 



Among those who believe the organ is or has been an eye, there are 

 important differences of opinion as to its present value. By Ahlborn 

 ('84), de Graaf, Spencer, and several other more recent writers, it is 

 believed to be degenerate and entirely functionless in all living verte- 

 brates. Rabl-Riickard ('86) has expressed the opinion that the organ 

 may still be of use in furnishing its possessors with a more delicate 

 means of detecting differences of temperature than exists elsewhere on 

 the body. Beraneck ('87) believes that, while the structure is probably 

 of an optical nature in some vertebrates, it has become so secondarily ; 

 that the primitive function of the epiphysis, common to the brains of 

 all vertebrates, was something entirely unknown to us now, though not 

 concerned with vision ; but that in the Cyclostomes, the Amphibians, 

 and the Reptiles it has taken on, secondarily, the function and form 

 of an eye. 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology, under the direction of E. L. Mark, No. XXII. 



VOL XX. — NO. 8. 14 



