Concerning the nature of these organs, Leydig 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 theory proposed by Leuckart, Ussow, and Leydig, and accepted by Semper 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 examinations 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, particularly in the jelly-tissues and the disposition of the nerve-endings. According 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. Leydig believes that two series of formations of this kind have been developed,
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 larvæ 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.