EYE OF POLYCYSTIS GOETTEI 467 



microns or less. Beyond this no further details of the pigmented 

 part of the eye could be recognized in the living condition nor 

 in sections that were too thick. In series of sections cut five 

 microns the cytology of the pigment cup could be worked out 

 fairly well. These sections show that the pigment mass of the 

 eye is unicellular. The nucleus of the pigment cell is oval and 

 has a large nucleolus. These two characteristics mark it off dis- 

 tinctly from the nuclei of adjacent cells, such as nerve and 

 mesenchymal cells (fig. 1, P-N). A bit of the cytoplasm is shown 

 about the nucleus in figure 1; but, for the most part the cyto- 

 plasm is completely obscured by the presence of many black 

 spheroidal pigment bodies (fig. 1, P, fig. 2, P). The cell body 

 of the pigment cell forms the wall of the pigment cup of the eye, 

 but a very striking feature of this cell body is that it for-ns a 

 conspicuous partition of pigmented cytoplasm, which, standing 

 up from the floor of the pigment cup, divides its lumen into two 

 secondary lumina. The accessory portion of this eye, therefore, 

 is a divided cup and presents two mouths or openings in- 

 stead of one. Thus in this eye there are two principal axes in- 

 stead of one as is usually the case. In short, here we have a 

 compound eye similar to the one described by Bohmig ('87). 

 Ordinarily one mouth of this compound cup is directed anteriorly, 

 while the other is directed posteriorly. These mouths open 

 dorsally as a rule. However, in our fixed material we have seen 

 specimens in which one opening of the pigment cup was ventral 

 to the other and the axis of each lumen was directed anteriorly. 

 Again, we have fixed material in which the mouths lie one be- 

 hind the other and the axes of the lumina are directed laterally. 

 This shifting of the axes of the lumina of the pigment cup in 

 fixed material is not to be considered as being due to torsion 

 resulting from fixing; but we believe that the animal has the 

 power to move its eye through a fairly wide range as it lies in 

 the parenchyma. This is perhaps due to the play of the ad- 

 jacent muscles of the proboscis. 



A single retinula which fills the lumen of each half of the pig- 

 ment cup, lies quite close to the dorsal ganglia so that it ap- 

 pears to arise out of the mass of cephalic nerve cells. The 



