ナマズのPit organ (孔器)

Abstract

In the present paper, I have examined the histological structure of the pit organs of a Japanese catfish. The pit organs, as HERRICK (1901) already mentioned, are divided into two kinds, viz., the large and the small pit organ. The large pit organs are commonly detected by the nakedeye examination on account of the absense of pigmentation around them. They are arranged on the head and trunk in difinite lines as shown if Fig. 1. The minute structure of the larege pit organ is illustraded in Figs. 2 and 3. Fig. 2 is a section through the pit organ observed most commonly. The specific sensory cells or pear-shaped cells crowd closely in the narrow outer portion of the organ. The sensory cells, as far as I could determine, seems to be deprived of the cilia and cupula. However, the indistinct coagulations are occasionally found over the exposed surface of the organ. I am incleined to think that such coagulation seems to be produced artificially by fixing reagents and presumably does not exist as a difinitely formed structure in the living organ. The large pit organ with such structure as above-mentioned was called “naked type” by HERRICK. In addition to this type, the more elaborate structure of the organ are found (fig. 3.) In this case, the organ sinks below the surface of the surrounding epidermis and is separated from it by circular groove. I wish to call such a structure under the name of the “sunken type” of the pit organ. According to HERRICK, the naked type seems to be an imperfetly developed stage of the sunken type, but as yet I could not ascertain this matter. Further research for the embryological study of this organ will help to throw ligth upon these points. The small pit organs, unlike the large ones, are not able to be located by the surface examination, although they are freely scattered over almost the whole body surfase. Fig. 4 shows the minute structure of this organ. The pear-shaped cells are found in this case, too. They are usually mmuch larger than, but similar to, those found in the large pit organs. In large specimens, the more complicated structure of the small pit organ is found (Fig. 5). The organ sinks below th surface of epldermis and communicates with surface by elongated, minute pore. Up to the present, I can not determine whether or not there is an intimate relation between these two structures of the small pit organs obves-mentioned.