1) The lateral-line endorgan of the fish in a living preparation could be observed in one of the species of sea eels, Rhyncocymba nystromi. The detailed structure by a phase contrast microscope showed that each sensory cell had a single short hair. These hairs formed a group over which a thin transparent membranous cupula extended so far that it approximately filled the cavity of the lateral-line canal.
This cupular structure has never been found in the histological preparation by an ordinary fixation method.
2) The actual motion of this cupula could be visualized under a microscope. It showed only the minimal deviation even by a pretty strong, artificial flow of the canal fluid.
3) The authors, contrary to the current conception on the mechanism of the lateral-line organ, believe that there is no free flow in the canal but that only the pressure change of the fluid can be transmitted to the endorgan through the lateral-line pores, which are conspicuously small or are plugged with the mucus of the body surface.
Such mechanism suggests also those of the vestibular or auditory organs of higher animals.
4) There are two kinds of fishes, one has a cupula terminalis at the crista acustica of the ear and the other only hair cells.
5) The intimate relationship between the lateral-line, the vestibular organ of the fish and the mucus was shown. These organs indicated a special staining character, the metachromasia, and were destroyed by an enzyme, a hyaluronidase.
These phenomena suggest the same relation in the chemical composition of the visual, auditory and vestibular organs of higher animals.