24 ARTHUR DENDT. 



above, before they come within range of the paired eyes, and 

 this I consider in all probability to be the function of this 

 organ. If the connection of the pineal eye with Reissner's 

 fibre, which I have suggested above, really exists, we may 

 further conclude that the eflBciency of the organ is greatly 

 increased by the " short circuiting" of the optic reflexes in 

 the same manner as has been described by Sargent in the 

 case of the ordinary paired eyes. 



As regards the parapineal organ, or left parietal eye, it is 

 more difficult to express an opinion. Here we have, in the 

 absence of pigment and of the projecting knobs of the sense 

 cells, evidence either that the organ has never attained so 

 high a degree of organisation as its fellow, or that it has 

 suffered degeneration, and similar evidence is afforded by the 

 much smaller size of the left habenular ganglion and the left 

 bundle of Meynert. The fact that it usually lies concealed 

 beneath the pineal eye also points to loss of function as a 

 light-perceiving organ; and it is interesting to note that in 

 this respect the genus Geotria, in which both organs are 

 exposed to the light, one in front of the other, appears to be 

 in a less degenerate condition than Petromyzon. The 

 retention of the well-developed connection of the parapineal 

 organ with the left habenular ganglion, however, seems to 

 indicate that it is still in some degree functional. It is difficult 

 to understand why one member of the original pair should 

 tend to degenerate any more than the other, but the degene- 

 ration itself may be connected with a possible greater effici- 

 ency of a strictly median organ in appreciating what is taking 

 place immediately above the animal. 



The paired origin of the parietal sense organs. — 

 The idea of a median, unpaired, Cyclopean eye on the top of 

 the head of the primitive Vertebrate ancestors has so struck 

 the popular imagination and become so firmly rooted even 

 in scientific literature that it is extremely difficult to gain 

 general acceptance for the modification of this somewhat 

 crude notion necessitated by modern research. Yet the 

 necessity for such modification confronts us at almost every 



