Ritter. — Parietal Eye. 



ABBREVIATIONS. 



Spot marking the position 

 of the parietal organ. 



External basement mem- 

 brane, [brane. 



Internal basement mem- 

 Nucleus. 



Nuclei of zone 2 of retina. 



Nuclei of zone 4 of retina. 



Nuclei of zone 5 of retina. 



Parietal bone. 



Pigment. 



Processes radiating from the 

 external basement mem- 

 brane. 



Retina. 



Blood sinus. 

 . Roof of the optic thalamus. 



Optic thalamus. 



Retinal blood-vessels. 



Third ventricle of brain. 



Epiphysial vesicle. 



Second zone of retina. 



Fig. 1. Left face of a section through the dorsal wall of the head of Phri/nosoma 

 Doucjlassii in the sagittal plane, and consequently passing through the 

 middle of the parietal organ. Diagrammatic in unimportant details. 

 X 140. 



" 2. Transverse section through the dorsal wall of the head and middle of 

 the parietal organ of P. coronata. Diagrammatic in unimportant de- 

 tails. X 140. 



" 3. Section of a small portion of the deep wall of the parietal organ and the 

 enveloping connective-tissue capsule, to show the processes radiating 

 from the external basement membrane. X 1060. 



" 4. A transverse section of one of the retinal vessels, in which a blood 

 corpuscle is seen. X 1060. 



