esterly: eucalanus. 41 



just as little reason for regarding them as "intraepithelial." Hesse's 

 definition of these two sorts of eyes is as follows : Entweder bleiben sie 

 [Sehzellen] in dem Verband des Epithels wie die indifferenten Epithel- 

 zellen, d. h. sie reichen mit ihrem distalen Ende ganz bis an die aussere 

 Begrenzung des Epithels .... dann sind sie selbst Epithelzellen 

 geblieben, wir bezeiehnen sie als epifhcliale Sehzellen. . . . Wenn 

 dagegen eine Sehzelle mit ihrem distalen Ende nicht bis an die distale 

 Grenze des Epithels reicht, iibrigens aber zwischen den Epithelzellen 

 liegt, so ist sie keine eigentliche Epithelzelle mehr, sie ist innerhalb 

 des Epithels gleichsam versenkt, proximad verschoben: wir bezeieh- 

 nen sie als intraepitheliale Sehzelle" (Hesse, :02, p. GIS, 619). 



From these definitions, it is plain that the ventral portion of the 

 median eye is to be regarded as composed of neither intraepithelial 

 nor subepithelial, but simply of epithelial retinal cells. I have shown 

 that it is continuous at its margins with the undifferentiated ectoderm, 

 and that it is, therefore, merely a thickening in the ectoderm. It 

 evidently is of the first t}^e, which Parker ('91, p. 59) has mentioned 

 as characteristic of most of the groups of Crustacea. Moreover, 

 Hesse (:02, p. 620) considers that the compound eyes of all arthropods 

 have epithelial visual cells. But it is impossible to regard the sensory 

 cells of the ventral part of the median eye as either intraepithelial 

 or subepithelial. 



x\s regards the cells of the lateral portions of the median eye, they 

 properly belong to those classed as subepithelial by Hesse or to the 

 third of the t^^es enumerated by Parker ('91. p. 60), if we are to judge 

 from the conditions in the adult. But Figures 6 (Plate 1) and 44 

 (Plate 5) show, especially at the anterior end, that the membranes 

 around the lateral eyes are a part of the basement membrane of the 

 hypodermis, though it cannot be claimed that the retinal cells and 

 those of the h}'podermis are now directly continuous as in the ventral 

 portion. As regards the membrane which surrounds the optic nerve, 

 its relation to the eye as a whole, and especially to the lateral portions 

 of the eye, seems to me to argue for the subepithelial character of the 

 retinal cells, for some of them certainly lie within the membrane, which 

 is plainly subepithelial in position. 



If, then, we attempt to place the median eye (as known in Eucalanus) 

 in the morphological classification proposed by Hesse (:02, p. 620), 

 it is necessary to consider that the unpaired portion occupies one 

 position in the system and that the paired portions occupy another. 

 The ventral ocellus is epithelial; the lateral ocelli are subepithelial. 

 And none of the cells of any part of the eye can be considered as in- 



