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ESTERLY: EUCALANUS. 45 
organs of Claus as having the function of eyes. Beer (:01, p. 13) and 
Hesse (:02, p. 610) have recently pointed out that pigment need not 
necessarily be present in order that light may stimulate an organ. 
And Helmholtz (56) long ago expressed the same view: “ Doch 
wissen wir durchaus nicht, ob alle pigmentirten sogenannten Augen- 
punkte der niederen Thierformén wirklich zur Lichtempfindung 
dienen. Andererseits miissen wir aus der Empfindlichkeit, welche 
niedere Thiere ohne Augenpunkte fiir das Licht zeigen, schliessen, 
dass auch lichtempfindende Nerven in durchsichtigen ‘Vhieren ohne 
Pigment vorkommen, die nur der Beobachter in keiner weise als 
solche erkennen kann.” 
Moreover, the great variability in the position of the pigment in 
eyes that must be regarded as homologous, as Hesse (:02, p. 613) 
has shown, is also against the view that a visual cell, as such, must be 
pigmented. 
It is unnecessary, and probably unwise, to speculate at length as to 
the manner in which the median eye in Eucalanus, or other animals 
with a similar organ, has been formed, because embryological evidence 
along that line is entirely lacking. I have already pointed out that the 
conditions in Eucalanus favor the view that the lateral eyes are sub- 
epithelial, having virtually lost their original position in the ectoderm. 
The simplest explanation for the adult condition seems to me to be 
that each of the vesicles forming the lateral eyes has become, by a 
simple revolution through 90° or more in opposite directions, so 
oriented that the ends of the cells which were at first ventral are now 
directed dorso-laterad. Thus in the paired eyes the ends of the cells 
which were originally covered by the basement membrane of the 
hypodermis have become, at least partly, reversed and are now directed 
medio-ventrad. This condition may be imagined as resulting from 
the rolling in of the lateral portions of a once-continuous sensitive 
area embracing all three of the components of the median eye, until 
the lateral margins of the area nearly meet in the median plane of the 
body, the middle ocellus undergoing in this revolution no change of 
position. 
But such alterations in the position of the eye as a whole as may be 
assumed to have taken place, have evidently not resulted in the forma- 
tion of structurally inverted cells in the lateral eyes. Claus (91, p. 
260) has assumed that as the eyes separated from the hypodermis there 
occurred “‘eine convergent nach einen Punkte gerichtete Drehung.... 
um eine Erklirung fiir das Zusammenstossen ihrer convexen Flachen 
und den Eintritt der Nerven von der Aussenseite in die Retina zu 
