PARIETAL SENSE-ORGANS OF GEOTRIA. 19 



been sufficiently described. Perhaps the most remarkable 

 difference which it exhibits as compared with its fellow con- 

 sists in the manner in which it is connected with the brain, 

 the organ itself lying immediately upon the anterior division 

 of the left habenular ganglion (figs. 2, 8, G.H.A.), while its 

 apparent nerve, the tract us habenular is of Studnicka 

 (figs. 2, 8, T.H.), is the long-drawn-out portion of the left 

 habenular ganglion which connects the anterior and posterior 

 portions of the latter. There is, therefore, strictly speaking, 

 no pi'oper nerve to the left parietal eye, which remains seated 

 immediately upon the brain, though no doubt the tr actus 

 habenular is functions as such. 



The parapineal organ of Geotria is a hollow sac, much 

 smaller than the pineal organ and of different shape, flattened 

 dorso-ventrally and elongated transversely (figs. 1, 4, L.P.^.). 

 It may be slightly, but distinctly, constricted in the middle 

 into right and left halves, or it may be more irregular in out- 

 line, as shown in fig. 4. The attachment to the anterior 

 division of the left habenular ganglion, though broad, does 

 not include by any means the whole of the ventral sui'face, so 

 that the pai'apineal organ is marked off from the ganglion by 

 a deep constriction, deeper in front and at the sides than it is 

 posteriorly. The outer surface of the organ is covered with a 

 thin sheath of connective tissue (fig. 8, C.T.S.) continuous 

 with the pia mater of the brain. 



The wall of the parapineal organ may be divided into 

 pellucida and retina exactly as in the case of the pineal eye 

 itself, but the distinction between the two is not nearly so well 

 marked. The pellucida (fig. 8, Pell.) consists of a layer of 

 columnar cells, the inner surface of which is in places drawn 

 out into irregular processes projecting into the cavity of the 

 organ exactly as in the case of the pineal eye, only in a less 

 perfectly developed condition. Outside these columnar cells 

 are numerous spherical nuclei, probably indicating a layer of 

 ganglion cells similar to those found in the retina. In the 

 pineal organ such nuclei are almost absent from the pellucida, 

 and in Petromyzon Studnicka describes the pellucida of 



