2O8 THE ASCIDIANS. 



and the subject of much speculation, one of the most cele- 

 brated hypotheses with regard to its significance being 

 that of DESCARTES, who regarded it as the seat of the 

 soul. 



More recently it has been shown to represent a rudi- 

 mentary, unpaired eye. Although in most cases, curiously 

 enough, it exhibits in existing forms no trace of an eye- 

 structure, it has been shown by DE GRAAF and SPENCER 

 that, as a matter of fact, in many lizards the distal vesicle 

 does actually become converted into an eye which, though 

 of a rudimentary character, is possessed of a retina, pig- 

 ment, and lens. In these forms the pineal body pierces 

 the roof of the cranium, occasioning the parietal foramen, 

 which is so characteristic of the Lacertilian skull, and the 

 pineal eye lies outside the cranium immediately below the 

 skin, through which it can be distinguished in external 

 view by the presence of a modified scale placed above it. 



In the animals below the lizards in the scale of organi- 

 sation (Amphibians and Fishes), as well as in those above 

 them, the distal vesicle of the pineal body apparently does 

 not become so far differentiated as to be recognised as 

 an actual eye, except in the case of the Cyclostome fishes, 

 where, as shown by BEARD, it presents the three essential 

 elements of an eye ; namely, retina, pigment, and lens, 

 lying, however, inside the cartilaginous cranium. 



The facts in our possession would seem to indicate 

 that the remote ancestors of the Vertebrates possessed 

 a median, unpaired, myelonic eye, which was subsequently 

 replaced in function by the evolution of the paired eyes. 

 It would, however, be premature either to assert this or to 

 express it as a definite opinion, especially since, in refer- 

 ring to the evolution of the paired eyes of Vertebrates, 

 we are bordering on ground upon which I have no imme- 



