EYE OF POLYCYSTIS GOETTEI 469 



for the red stain. This is especially the case with material fixed 

 in chrom-aceto-formalin. This region, which we take to be 

 homologous with the 'refractive body' which Kepner and Telia- 

 ferro ('16) described for the eye of Prorhynchus applanatus, is 

 wedge-shaped. This region has been recorded for a triclad 

 rhabdome, and a function suggested for it by Taliaferro ('17) 

 who says, 



The rhabdome itself shows an optically denser region in its outer 

 end as described in Prorhynchus applanatus by Kepner and Talia- 

 ferro ('16). This region because of its shape and density, must have 

 some effect upon the rays of light if they pass through the long longi- 

 tudinal axis, which it cannot have if they pass through in any other 

 direction. 



The wider faces of the denser wedge-shaped region of the 

 rhabdome of Polycystis goettei are parallel to the partition of 

 the pigment cup (fig. 5, E) and their apices are directed towards 

 the fundus of the lumina of the cup (fig. 2, E and fig. 3). In 

 brief, we may say that there are three regions clearly differentiated 

 in the retinula of this rhabdocoele: (a) a proximal region 

 that bears the nucleus; (b) a distal region, which is the end 

 organ or rhabdome; and (c) a middle region arising from the dis- 

 tal end of the basal part of the cell and extending into the cone- 

 shaped rhabdome. 



This three-fold differentiation of the retinula of Polycystis 

 goettei is homologous with the retinula of Prorhynchus applanatus 

 where there is a basal region which bears the nucleus and is 

 homologous with a similar region in the retinula of this rhab- 

 docoele (fig. 4, A and A'). The second region of this cell of 

 Prorhynchus applanatus has been seen in fresh material compressed 

 between cover-glass and slide to be highly refractive. In the 

 retinula of Polycystis goettei (Bresslau) we have a region homol- 

 ogous with this refractive region (fig. 4, B and B'). Finally 

 the end organs in the two retinulae of the two rhabdocoeles are 

 homologous (fig. 4, C and C). 



Thus we have homologous regions in the eyes of the two 

 rhabdocoeles, which are strikingly analogous to, if not homolo- 

 gous with, the three regions that have been described in the 



