40 bulletin: museum of comparative zoology. 



sind die Medianaugen nichts Anderes als die iibrig gebliebenen 

 Naiipliusaugen, die eine Erbschaft von walirscheinlicli plathel- 

 minthenartigen Vorfahren darstellen." 



The essential point in all these comparisons is the inverted character 

 of the retinal cells of the median eye; it is maintained that the axis 

 cylinders, which are made up by the union of neurotibrillae from 

 within the cells, leave the cells from that part which is directed away 

 from the pigment. But, if the eye of Eucalanus may be taken as the 

 type of the so-called persisting nauplius eye, such a comparison as 

 that instituted by Hesse must fail, as well as the conclusions deduced 

 from it. Hesse himself (:01, p. 350) has considered the median eye 

 in Eucalanus as of typical form, as did Grenadier ('79, p. 63) and 

 others who followed him (Carriere, '85; Lang, '88-94); in the ab- 

 sence of trustworthy evidence to the contrary, it seems to me that 

 such a median eye may fairly be taken as representing the general 

 form of the tripartite eye as found among Crustacea. At any rate, 

 the median eye is a more characteristic structure in the Copepoda 

 than in any other group, and, among the Copepoda, the eye of 

 Eucalanus has been more adequately studied than that of any other 

 genus. 



But so far, then, as my observations extend, there is no evidence 

 from the manner of innervation of the median eye that it is of the 

 inverted type. For, as Beer (:01, p. 12) has said of the uninverted 

 eye "das Licht unter den gewohnlichen Bedingungen erst die Photir- 

 zelle, dann den Opticusabgang trifft." 



The median eye of Crustacea has been jjlaced in the same class 

 with those of the flat-worms, and of many annelids, on account of its 

 position as regards the epithelium. Hesse (:02, p. 620), in a table 

 giving the results of his investigations, includes the median eye of 

 Crustacea among those whose visual cells are subepithelial. This 

 term is defined (Hesse, :02, p. 619) as follows: "Wenn dieser gleiche 

 Vorgang, der eine ursprlinglich e]:)itheliale Sehzelle zur intrae])ithe- 

 lialen werden liisst, noch weiter fortschreitet, so verliisst die Sehzelle 

 den Bereich des Epithels vollkommen: sie wird zur snhepitheUalen 

 Sehzelle.'' Claus ('91, j). 260) is also of the opinion that the median 

 eye as a whole is separate from the hypodermis. I believe my results 

 prove that only the paired portions of the median eye can be regarded 

 as subepithelial in the sense in which Hesse has used the term. 

 There is very little indication in the adult condition that the lateral 

 eyes are a part of the general ectod(>rm of the body; consequently 

 they cannot belong to Hesse's class of epithelial eyes, and there is 



