Herrick, Werves of Siluroid Fishes. 233 
neither elevated nor depressed at that point, and the pore may 
run downward as a straight tube of considerable length before 
dilating into the sac-like cavity of the pit. Glandular, or 
mucous, cells and the clavate cells of Leydig are found in close 
proximity to the pits, as in the case of the terminal buds, and 
I have not been able to locate these small pits by surface ex- 
amination of either fresh or preserved specimens of any species. 
These pits are exceedingly numerous and tend to be ar- 
ranged in rows or lines, though this arrangement is usually not 
noticeable. To map them accurately would be very laborious 
and I have not attempted it. In very young specimens this 
will doubtless prove not only easy, but much more fruitful in 
morphological suggestion. They are freely scattered over 
almost the whole body surface, but much more plentiful on the 
head than on the trunk. They are in general perhaps more 
abundant in the neighborhood of the canals and lines of large 
pit organs, but not conspicuously so. Their nerve fibers are of 
medium size, but their medullary sheaths are exceedingly feebly 
developed, so that the innervation of the organs can be traced 
with great difficulty, even in good WEIGERT sections. 
The organs which I here term large pit organs resemble 
quite closely those which I called pit organs in my accounts of 
Menidia and the young cod fish which formed the subjects of 
my previous memoirs. These organs genegally appear in my 
sections to be strictly superficial, usually projecting slightly — 
above the surrounding surface, and they are commonly very 
conspicuous upon surface examination by reason of the absence 
of pigmentation around them. They rest on unmodified corium 
or the dermis may be slightly elevated under them, especially 
around their edges, thus forming a sort of saucer-shaped eleva- 
tion upon which the cells of the sensory organ rest. The shape 
of the organ is that of a broad dome with a flat base which 
rests directly upon the chorium. The epidermis becomes thin- 
ner as the organ is approached and its margin overlaps the base 
of the organ, a shallow circular groove on the surface marking 
the contact of the non-sensory epidermis with the sense organ. 
The organ projects above the surface of the surrounding epi- 
