22 bulletin: museum of comparative zoology, 



seems to me to be indicated by the innervation of the retina. The 

 facts concerning the latter have been rather confused, if one considers 

 all the accounts in the literature. In general the manner of innerva- 

 tion has been looked upon as a factor in determining the phylogenetic 

 relationship of the Copepoda. 



Grenacher was the first to determine the way in which the axis 

 cylinders are related to the retinal cells. He says (Grenacher, '79, 

 p. 6.5) that the fibres of the optic nerve "are continuous with the inner, 

 pointed ends of the cells," meaning the ends nearer the pigment plate. 

 He was able to follow the nerve fibres into the cells. 



Following Grenacher's work, came that of Hartog ('88, p. 3.3), who 

 found that in Cyclops the ocellus receives the nerve posteriorly at the 

 outer surface, so that the optic elements are reversed as in the flat- 

 worm Dendrocoelum. The partition which separates the eye from the 

 brain is "quite imperforate by nerves." Further on (p. 34) he states 

 that in both Cyclops and Calanus he followed a few fibres along the 

 septum between the blocks of the lateral ocelli, and had positive evi- 

 dence that such do not enter the "bacilli"; they may end in the nuclei 

 of the blocks or pass on to the frontal region. And in a footnote (p. 34) 

 he describes the results of dissecting and sectioning the eye of Calanus 

 in alcoholic specimens; he there found that "the lateral branches [of 

 the optic nerve] unquestionably do not enter the inner ends of the 

 bacilli"; he was unable to speak with certainty about the ventral eye. 



As already stated, Claus ('91) did not confirm Grenacher's observa- 

 tions as to the point at which the optic nerves leave the cells of the eye. 

 Claus was evidently of opinion that the nerve leaves from the outer 

 side of the visual cells and that the recipient ends of the fibrils are turned 

 toward the pigment body, thus making the median eye an "inverses 

 Becherauge." He found this to be the case in the Cypridinidae, 

 Branchiopoda, Cladocera and Argulidae and implies that it is so in 

 the Copepofla and Cirripedia. 



Richard ('91, p. 208) also found that the retinal elements are "ren- 

 versfe comme dans les yeux marginaux des Pecten et dans celui des 

 Vertebres," for the optic nerves do not enter the pigment mass, but 

 pass to the dorsal margin of each simple eye and terminate at the 

 surface. 



Other investigators are more or less inclined to consider the parts of 

 the median eye as inverted. Schmeil ('97, p. 30) accepts the state- 

 ment that such is the case. Carriere ('85, p. 178) leaves the question 

 open, but considers the resemblance of the eyes of Calanella (Eucala- 

 nus) to those of Clepsine or Planaria "as unmistakable." Lang 



