20 ARTHUR DENDY. 



the parapiueal organ as consisting of only a single layer of 

 cells. 



The pellucida passes quite gradually into the retina, which 

 consists of an epithelial layer of columnar cells facing the 

 cavity of the organ and backed by a nervous layer of ganglion 

 cells and nerve fibres. Thus the retina has a close general 

 similarity to that of the pineal organ, from which, however, 

 it differs strikingly in the entire absent of pigment. Accord- 

 ing to Studnicka the retinal epithelium of the parapineal 

 organ in Petromyzon consists of sensory cells and ordinary 

 ependymal cells, but I have not been able to distinguish 

 clearly between the two in Geotria. As in Petromyzon, 

 the characteristic knob-like projections of the retinal cells 

 into the cavity of the organ, which are so conspicuous in the 

 pineal eye, are not to be found in the parapineal. 



In the interior of the parapineal organ we find, exactly as 

 in the pineal, a network of delicate threads connecting the 

 pellucida with the retina (fig. 8). Here again this network 

 appears to be formed by outgrowth of the columnar cells of 

 the pellucida, and contains small nuclei scattered in it. 



The nervous layer of the retina must be considered in 

 connection with the underlying anterior division of the left 

 habenular ganglion (fig. 8, G.H.A.). This consists of a 

 central mass of finely granular or punctate matter devoid of 

 nuclei, but partially surrounded by nerve cells, as shown in 

 longitudinal section in fig. 8. In transverse sections (fig. 9) 

 the central mass is seen to extend laterally in a pair of hori- 

 zontal, wing-like projections, beneath which the nerve cells 

 are accumulated. From the upper surface of the central 

 mass stout bands or tracts of fibres are given off, which 

 curve upwards amongst the ganglion cells of the retina, and 

 sometimes appear to extend even into the pellucida. These 

 fibrous bands can, in part at any rate, be traced directly 

 backwards into the tractus habenular is, as shown in 

 fig. 8. 



The anterior division of the left habenular ganglion passes 

 backwards quite gradually into the tractus habenular is. 



