THE MEDIAN VERTEBRATE EYE. 129 



tlie huge extinct reptiles, suoh as Ichthyosaurus and 

 Plesiosaurus, it has been, I think, fairly inferred that 

 the pineal eye was much more developed than in any 

 known living form. 



In living fish and Amphibia, so far as they have been 

 yet examined, the organ is even more rudimentary 

 than in reptiles. But in the fossil Labyrinthodonts the 

 skull possesses a large and well-marked orifice for the 

 passage of the pineal nerve. This orifice is, in fact, 

 so large that it can scarcely be doubted that the eye in 

 these remarkable amphibia was also well developed, 

 and served as a third organ of vision. 



In birds the organ is present, but retains no re- 

 semblance to an eye. It is solid and highly vascular. 

 In mammals it is still more degenerate, though a trace 

 is still present even in man himself. 



The larval Ascidians, which present so many points 

 of resemblance to the lowest vertebrates, and especially 

 to the Lancelet (Amphioxus), have hitherto been re- 

 garded as differing from them in the possession of a 

 central eye. It now, however, appears that the verte- 

 brate type did originally possess a central eye, of which 

 the so-called pineal gland is the last trace. 



It seems, then, very tempting to regard the pineal 

 eye as representing the central eye of Amphioxus; 

 but Spencer points out that the two organs differ 

 greatly in structure, and he himself doubts whether 

 the pineal eye is really the direct representative of the 

 central eye in the Tunicata. 



Beraneck * also regards tlie pineal as entirely 

 different from the central eye of the Tunicata. Indeed, 

 he considers its differentiation as an eye to be a 



* " Ueber d. Pariutal Auge der Reptilien," Jenaische Zeit., 1887. 



