PARIETAL SENSE-ORfiANS OP GEOTRTA. 5 



lined by an epithelium composed of very much elongated 

 columnar cells. This groove is evidently formed by union 

 of the pair of grooves which I described in the Ammocoete 

 in 1902 (5) ; it has since been termed by Sargent (6) the 

 '^ependymal groove." In the "Velasia," though the two 

 grooves are closely approximated, they still show clear indi- 

 cations of tlieir double origin (fig. 3^ Ej), G.). Theependymal 

 groove is continued forwards into the recessns infra- 

 pineal is, nnd thence for a short distance beneath and to 

 the left of the right habenular ganglion, gradually losing 

 the special character of its epithelium. 



According to Sargent (6) the epithelium of the ependymal 

 groove serves for the support and attachment of the anterior 

 branches of Eeissner's fibre on their way from the optic reflex 

 cells to the brain cavity, in which the main fibre lies freely. 

 From my own observations on Geotria I have come to the 

 conclusion that the anterior constituent fibrils of Reissner^s 

 fibre (fig. 2, R.F.) do leave the brain substance in the 

 ependymal groove as described by Sargent, but this discovery 

 by no means disproves the view which I previously (6) put 

 forward with regard to the function of the ependymal grooves 

 in the Ammocoete. Reissner's fibre itself is very conspicuous 

 in Geotria, as shown in fig. 2, i?.i^., but further discussion of 

 this part of the subject may be conveniently left until later on. 



(c) The Pineal Organ (Right Parietal Eye). 



General form and structure. — The structure of this 

 organ (fig. 7) is in its general features very similar to that 

 described by Studnicka for the European lampreys. It con- 

 sists of a hollow vesicle, about half a millimetre in maximum 

 diameter, and having a very characteristic shape, not unlike 

 that of a pear, with the pineal stalk representing the stalk of 

 the pear. The upper surface of the optic vesicle, turned 

 towards the light, is perfectly circular in outline and very 

 much flattened like a button (fig. 4), while the lower surface 

 is strongly convex, especially posteriorly, where the wall of 



