PARIETAL SENSE-OT?GANS OF GEOTBTA. 9 



of an outer layer which stains less deeply with acid fuchsin. 

 Possibly this is the explanation of the appearance described 

 by Studnicka. In other respects the sensory cells appear to 

 be quite identical witli those of Petromyzon. 



The intervals between the sensory cells are filled by the 

 pigment cells, which Studnicka has, no doubt correctly, 

 identified with the ependymal cells of the general internal 

 lining of the brain cavities. The pigment cells (figs. 5, 7) are 

 broadest at their inner ends, next to the lumen of the optic 

 vesicle, and taper gradually outwards till their slender, 

 thread-like outer extremities, which may apparently branch, 

 are lost in the fibrillar network of the nervous layer, along 

 with the outer ends of the sensory cells. 



So far the pigment cells agree fairly well with those of 

 Petromyzon, as described by Studnicka (2), but there is 

 one very important difference. In Petromyzon it appears 

 that they terminate at their inner extremities in a smooth 

 surface, through which the knobbed ends of the sense-cells 

 project, while, according to Studnicka's latest account, they 

 themselves have no differentiated inner segments or knobs at 

 all. In Geotria, on the other hand, the inner end of each 

 pigment cell (fig. 5, 1.8. P.C.) is very distinctly segmented off, 

 and separated from the outer and principal portion of the 

 cell {O.S.P.C.) by what looks like a limiting membrane {L.M.). 

 In depigmented sections, as shown on the left-hand side of 

 fig. 5, this "limiting membrane^' is very conspicuous, and 

 appears, at first sight, to form the inner surface of the retina; 

 it has a characteristic dotted or beaded appearance. Careful 

 observation shows, however, that even in depigmented sections 

 the remains of the inner ends of the pigment cells, as well 

 as the projecting knobs of the sense cells, may be clearly 

 recognised as the inner side of the " limiting membrane," 

 though not neai'ly so conspicuous as in sections in which the 

 pigment is preserved, as shown on the right-hand side of 

 fig. 5. _ 



Thus in Geotria the pigment cells as well as the 

 sense-cells are provided with differentiated, knobbed inner 



