Eyes of MoUuscs and Arthropods. 687 



tlie rods and histological structure of the retina would necessarily 

 undergo deg-eueration. and no longer function as sense organs; 

 lienee their nervous supply would almost entirely disappear, a 

 eondition whieh has, apparently, not been realized. But the diffìculty 

 in the way of such a sup])osition is to point out any purpose in such 

 a chauge. or any adequate advantag-e to be gained by the animai; 

 and then we have only doubled the difficulties, since we must still 

 explain how so many accessory eyes were originally developed. 



In fact, to suppose that the orgaus are luminous does not decrease, 

 but on the eontrary augments the difficulties to be met in treating this 

 subject, therefore I caunot agre e with Öars in considering them as 

 luminous organs, but am compelled to regard them as accessory 

 eyes. It seems to be beyond doubt that they are luminous; but still 

 I desire to call attention to the fact that the proof does not as yet ap- 

 pear to be perfect. For instauce, Sars has not said that the eyes are 

 luminous in absolute darkness! It must be borne in mind 

 that the presence of the brilliaut argentea would, in certain positions, 

 cause a sudden reflection of any bright object in the vicinity, and thus 

 produce the brilliaut flashings which Sars considers voluntary. 

 But in spite of these facts, it appears that the rods, or fibres, are 

 faintly luminous, even when isolated and crushed. But even this does 

 not prove that the structures are luminous organs, since this property 

 is a very common one, found in almost all couditions of matter, living 

 or dead, and in the present iustance it could very easily be an inci- 

 de ntal property, haviug nothing at all to do with the use of the 

 organs. I would also like to call attention to the presence of the 

 bright red pigment so characteristic of certain forms of Inverte- 

 brate eyes i. e. Ccelenterata, Echinoderms, Molluscs and Worms. 



If these bodies in Eup/musiasiVG not »luminous organs« — that 

 is, if the luminosity is not an essential, but only a secondary and inci- 

 dental property, — then they are, in all probability, visual organs, or 

 eyes. But how can we explain the presence of so many eyes in this 

 isolated group' It is probably due to one of two reasons, either they are 

 deep sea, or nocturnal animals. But from the «Challenger Reports«, 

 it is evident that they are found in great abundance on the surface, 

 and seldom. or never, at any depth. They are, probably then, noct- 

 urnal , and this fact has led to a great development of ommatidia in 

 certain regions , giving rise to the eyes as we now see them. The 

 enormous development of the argentea is an evidence that they are 

 constructed for an economy of light impressions, just as in Pecteti, and 



