PARIETAL SENSE-ORGANS OF GEOTRIA. 25 



point of view, morpliolog-ical, embryological,and even palfeon- 

 tological. In this connection we cannot content ourselves 

 with the consideration of any one Vertebrate group, but must 

 seek for evidence from as wide a field as possible. I 

 discussed the problem at some length in my memoir (3) 

 " On the Development of the Parietal Eye and Adjacent 

 Organs in Sphenodon (Hatteria)/' published in this 

 'Journal' in 1899, with special reference to the Tuatara. Since 

 that date the embryological investigations of Cameron have 

 afforded striking confirmation of the views then adopted, and 

 my researches on the New Zealand Lamprey, described in 

 the present memoir, strongly confirm me in the opinion that 

 the so-called pineal and parapineal organs represent the 

 right and left members of a primitive pair. 



The evidence derived from the study of Geotria may be 

 summarised as follows : 



(1) The parapineal organ, in its position to the left of the 

 pineal, still shows evidence of its primitive paired character. 



(2) The structure of the pineal and parapineal organs is 

 essentially identical, although the former is much more 

 highly developed than the latter. 



(3) The connection of each of the two sense organs with 

 the corresponding member of the habenular ganglion pair 

 need no longer be questioned, 



(4) The marked asymmetry in point of size of the two 

 habenular ganglia, and of the two bundles of Meynert, 

 corresponds exactly to the unequal development of the two 

 parietal sense organs with which they are connected, and 

 leaves no doubt as to the paired character of the whole system. 



The embryological investigations of Cameron (8, 9) confirm 

 strongly the general results obtained by Hill, Locy, and 

 myself. In Amphibia and in the chick Cameron shows that 

 the " epiphysis " is in origin a bilateral structure, just as 

 Hill had shown for Teleosteans, Locy for Elasmobranchs, and 

 the present writer for Sphenodon. In some cases, including 

 man, however, Cameron (10) maintains that there is a decussa- 

 tion of the nerve fibres at the base of the epiphysis, each lateral 



