PAKIKTAL SENSE-ORGANS OF GEOTRIA. 13 



(d) The Pineal Nerve and its Connections. 



It is well known that in Petromyzon the so-called 

 "pineal outgrowth" arises immediately in front of the pos- 

 terior commissure^ and grows forward above the roof of the 

 fore-brain in the form of an elongated hollow sac, whose 

 distal extremity enlarges and becomes modified in structure 

 to form the pineal or parietal eye, while the proximal por- 

 tion, or "stalk/' becomes solid, and by histological differen- 

 tiation is, in part at any rate, converted into the pineal nerve. 



In Geotria, as in Petromyzon, the original point o£ 

 connection of the pineal stalk with the brain is clearly indi- 

 cated by the depression between the posterior commissure and 

 the right habenular ganglion known as the re cess us infra- 

 pinealis, as shown in figs. 2 {R.I.P.) and 6. At this spot 

 the epithelium of the ependymal groove, in sections, is usually 

 pulled out and separated from the rest of the ependymal epithe- 

 lium owing to the inevitable contraction in preparation, while 

 remaining closely adherent to the pineal stalk above it, to 

 which it is intimately attached by fibres which appear to belong 

 to the pineal nerve. This connection of the epithelium of the 

 ependymal groove with the pineal nerve has not, so far as I 

 am aware, been hitherto observed, and appears to me to be 

 a matter of considerable interest, though it must not be 

 forgotten that some at any rate of the connecting fibres may 

 be merely connective tissue. 



The pineal stalk in Geotria is not, as a whole, very sharply 

 defined, but merges on either side in the mass of arachnoid 

 tissue which lies outside the brain. It thus appears much 

 more definite in longitudinal than in transverse sections, 

 forming a solid cord, apparently of loose connective tissue 

 (figs. 1, 6, P.S.), in which the pineal nerve itself is 

 imbedded. This nerve consists of a bundle of numerous 

 very slender, non-meluUated fibres, containing elongated 

 nuclei, and indistinguishable from those of higher vertebrates, 

 as represented, for example, in fig. 138 of Schafer's 'Essentials 



