PAEIETAL SENSE-ORGANS OF GEOTRTA. 23 



Stuclm'cka, T fiud it impossible to believe that, in the Lampreys, 

 it is not at the pi'esent da}^ functional. It exhibits, in my 

 opinion, no sign of degenei^ation; sense cells, pigment cells, and 

 ganglion cells are all present in a high degree of perfection, 

 and the retina is connected with the brain by a well-developed 

 nerve. The enormous development of the right habenular 

 ganglion and of the right bundle of Meynert, with which the 

 pineal nerve is connected, nlso clearly indicate functional 

 activity on the part of the pineal eye. Perhaps the most 

 striking evidence in favour of this view, however, is afforded 

 by the modification of the overlying tissues to form a light- 

 transmitting apparatus. It is one of the fundamental axioms 

 of biology that disuse leads to degeneration, and we may 

 safely assume that a high degree of structural differentiation 

 implies a corresponding degree of functional activity. 



Everything points to the fact that the function of the pineal 

 organ is that of light perception, and therefore we are justified 

 in speaking of it as an " eye." Its structure, however, 

 especially in the Lampreys, differs in important particulars 

 from that of any other eye known to ns. In the Lampreys, 

 at any rate, it is not, as Studnicka has already pointed out, 

 a cameral eye, and we cannot suppose it to be capable of 

 forming an image. There is nothing which we are justified 

 in regarding as a lens, and the peculiar nature and arrangement 

 of the " white" pigment is calculated to reflect the raj-s of light 

 in every direction, and thereby prevent the formation of an 

 image, even if the necessary dioptric apparatus were present. 



On the other hand, it may well be that the brilliant white 

 pigment, by reflecting the light rays npon the knobs of the 

 sense cells, may thereby serve to intensify the light stimulus 

 and render the whole organ extremely sensitive to the 

 variations in the intensity of the illumination to which it 

 is exposed.^ Such sensitiveness might be of great value 

 in giving timely warning of the approach of enemies from 



' This view of tlie function of the knobs of the sense cells is totally 

 opposed to that of Studnicka (12), who regards them as so many independent 

 lenses, each serving to focus the light upon its own particular sense cell. 



