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THE POPULAR SCIENCE MONTHLY. 



Concerning the nature of these organs, Ley dig denies that any of 

 them are glandular, although Ussow admits that this may be the case 

 with some of the fishes. The hypothesis that they are organs of a 

 sixth sense has received no confirmation. There remains, then, the the- 

 ory proposed by Leuckart, Ussow, and Leydig, and accepted by Sem- 

 per as undoubtedly correct, that they are real subsidiary eyes, like 

 the eyes of mussels, etc. Leuckart and Ussow believed that they 

 were able to distinguish a lens, a vitreous substance, and a retina, and 

 the latter has published drawings of those parts ; but the careful ex- 

 aminations of the structure of the organs and comparisons between 

 it and the eyes of mollusks have led Professor Leydig to doubt this 

 opinion ; for he has observed that, when the fish swims horizontally, 

 the mouths of the supposed eyes are turned, not toward the light, but 

 downward, toward the dark bottom. Still less do the glass-pearly 

 organs resemble eyes. Leydig is rather disposed to believe that he 

 can with great probability recognize an identity in their structure with 

 that of the electric and pseudo-electric organs of some fishes, particu- 

 larly in the jelly-tissues and the disposition of the nerve-endings. Ac- 

 cording to this view, each of the disks would in itself correspond to a 

 chest of the electric organs. The round shape of the disks may be 

 explained by their isolated situation, there being no pressure of one 

 upon another to make them angular. A similar diversity prevails in 

 the form of the electric and pseudo-electric organs to that existing in 

 the organs which we are considering, while the homology of the two 

 is strikingly expressed in their similar situation and distribution. Ley- 

 dig believes that two series of formations of this kind have been devel- 



Fig. 5. Two " Glass-pearly " Organs from the Side of Scopelus Humboldtii, moderately 



magnified. 



oped, one of which leads through the pseudo-electric organ of the 

 Gymnarchus niloticus and the disk-like organs of the Scopelids to the 

 real electric organs, while the other series includes the eye-like organs 

 of the sternoptychids ; an apparatus which is also represented in the 

 larvse of salamanders. 



The appearance of this phenomenon in the amphibia, frequently 

 observed as they approach the fish type, should point to some definite 

 connection between the activity of those organs and water-life ; but 

 the nature of this activity, whether electricity is developed by it or 

 not, is still veiled in complete darkness. 



