234 JouRNAL OF COMPARATIVE NEUROLOGY. 
dermis by about one-fourth of its thickness. The nerve pene- 
trates the corium under the middle of the organ, its fibers los- 
ing their medullary sheaths and spreading out over the lower 
surface of the sense organ. The specific sensory cells, or pear 
cells, are crowded rather closely in the narrow outer portion of 
the dome-shaped organ and they extend inward about one-third 
of the thickness of the organ. The exposed surface of the 
organ is covered with a dense external limiting membrane 
which is continuous with that of the surrounding epidermis and 
upon which is a more or less indistinct coagulum which may re- 
present vestiges of cilia. This is the type of large pit organs 
most often observed in my sections of Ameiurus; but some of 
larger organs present a more elaborate structure. 
In these cases the organ is sunken below the surface of the 
surrounding epidermis and separated from it by a rather deep 
circular groove, some modified epidermal cells on inner side of 
the groove arching in the form of a lip over the edge of the 
free ends of the sensory cells. Fig. 13 shows a section taken 
somewhat to one side of the middle of such an organ. Ina 
section taken through the middle of the organ, the patch of 
sensory epithelium is considerably wider and a proportionately 
smaller part of the exposed surface is overlapped by this lip. 
The nerve is there likewise seen to pierce the corium and enter 
the center of the organ very much as it appears in the small 
pit organ, Fig. 12. The nerve fibers for the large pit organs, 
unlike those for the small pit organs, are large and densely 
medullated, appearing in the sections much like those for the 
neuromasts in the lateral line canals. : 
Surface examination with a lense of the larger specimen of 
A. melas shown on Fig 14 gives a quite different impression of 
these organs. They appear as slightly elevated spots rather 
conspicuous by reason of the total lack pigment. They are 
rarely circular, but more often a raised line runs out at opposite 
ends of a diameter of the disc as shown in the figure. When 
the organs are arranged in definite lines they are frequently con- 
nected by a continuous raised line in this way. Commonly no 
pore is visible at the apex of the papilla in alcoholic specimens, 
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