PARIETAL SENSE-ORGANS OF GEOTRIA. 29 



dividing the saccus vasculosus into right and left halves. Sp.C. Spinal cord. 

 T.H. Tractus liabeuularis. Thai. Thalamenoephalon. T.O. Tectum opticum. 

 F». Third ventricle. F'*. Pourlh ventricle. X. Point where the bundles of 

 Meynert reach the base of the brain. 



(All the figures refer to Geotria australis [Velasia].) 



PLATE 1. 



]7xG. 1. — Brain; dorsal view after removal of the choroid plexuses of the 

 mid- and hind-brain, x 10. 



Fig. 2.— Sagittal section through the brain, with the surrounding cranium 

 and overlying structures (slightly diagrammatic, and with the arachnoid 

 tissue omitted). 



Fig. 3.— Transverse section of the brain in the region of the posterior 

 commissure, showing the ependymal groove, bundles of Meynert, etc. Drawn 

 under Zeiss A, oc. 2. 



PLATE 2. 



Fig. 4. — The pineal and parapineal organs (right and left parietal eyes), as 

 seen from above under a dissecting lens. 



Fig. 5. — Diagram showing the structure of the retina of the pineal organ 

 (right parietal eye). 



Fig. 6. — Longitudinal vertical section through the recessus infrapinealis, 

 showing the relation of the pineal nerve to the posterior commissure, the right 

 habenular ganglion, the right bundle of Meynert, the ependymal groove, and 

 Reissnor's fibre (slightly diagrammatic). 



Fig. 7. — Sagittal section of the pineal organ (right parietal eye), drawn 

 under Zeiss D, ocular 2 (slightly diagrammatic). 



Fig. 8. — Longitudinal vertical section through the parapineal organ (left 

 parietal eye) and the anterior portion of the left habenular ganglion. Drawn 

 under Zeiss D, oc. 2 (slightly diagrammatic). 



Fig. 9. — Transverse section through the hinder part of the anterior portion 

 of the left habenular ganglion, immediately behind the parapineal organ. 

 Drawn under Zeiss D, oc. 2. 



