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 Thierformen wirklich zur Lichtempfindung 

 dienen. Andererseits miissen wir aus der Empfindlichkeit, welche 

 niedere Thiere oline Augenpunkte flir das Licht zeigen, schliessen, 

 dass auch lichtempfindende Nerven in durchsichtigen Thieren ohne 

 Pigment vorkonimen, 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. G13) 

 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 eve 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 h_\'podermis there 

 occurred "eine convergent nach einen Punkte gerichtete Drehung .... 

 um eine Erklilrung flir das Zusammenstossen ihrer convexen Flachen 

 und den Eintritt der Nerven von der Aussenseite in die Retina zu 



