PARIETAL SENSE-ORGANS OF GEOTRIA. 6 



of the cavity of the saccus vasculosus iuto right and left 

 halves by a well-developed long-itudinal septum (fig-. 2, Sept.) 

 deserves mention. 



The thalamencephnlou lias, in its anterior part, a thin 

 membranous roof which rises upwards in a prominent dome. 

 This dome lies immediately beliind and between the olfactory 

 lobes, and the thin roof bends down in front to form the 

 lamina terminalis (fig. 2, L.T.). On this thin dome- 

 shaped roof of the third ventricle lie the organs with wliicla we 

 are more immediately concerned, the pineal or parietal sense- 

 organs. There are in the Lampreys, as is well known, two of 

 these sense-organs, and in the genus Petromyzon one lies 

 beneath the other, the upper one being by very much the 

 better developed of the two, and being commonly spoken of 

 as the "pineal eye." In the terminology of Studuicka the 

 upper one is described as the " pineal organ," and the lower 

 one as the "parapineal organ." According to the view adopted 

 by myself (3), and long since maintained by Gaskell (4), these 

 two sense-organs are really members of a pair which have 

 become displaced, the upper and better developed repre- 

 senting the right "parietal eye," and the lower the left one. 

 This view is supported in a very interesting manner by the 

 arrangement of the two organs in Geotria. It will be seen 

 from figs. 1 and 2 that the larger and better developed of the 

 two does not lie above but behind the smaller and less well- 

 developed organ, so that both are distinctly visible when the 

 brain is viewed from above. Moreover, I find in all cases 

 where the organs have been carefully examined in situ, 

 that the anterior one lies a little to the left side of the pos- 

 terior, the latter being approximately median in position. 

 This clear indication of the paired origin of the parietal 

 sense-organs affords a close parallel to the condition described 

 by myself in embryos of Sphenodon (3). In both cases there 

 is an anterior parietal organ lying immediately in front of and 

 a little to the left of a posterior one, but in Sphenodon, 

 curiously enough, it is the left (anterior) organ which becomes 

 well developed as the apparently unpaired "pineal eye" of 



