GENERAL DISCUSSION. 17 
so far as we know, has no parallel among the fishes unless it may be to a re- 
mote extent in the case of Ipnops, a genus belonging to avery different 
family. The floor of each orbit against the skull, is lined by an extensive 
sheet of silvery tissue, primarily the iris, so broad as to nearly meet its fel- 
low from the opposite side on the top of the forehead. This concave lining 
directed forward and upward, and to some extent toward the side, probably 
in part serves asa reflector, but it contains a small spot of black pigment, 
a little backward of its middle, that may retain something of the retinal 
function. The cavity appears, from the alcoholic specimen, to have been 
filled with liquid kept in place by the thin transparent outer covering. 
Crude as the organs appear they no doubt served as eyes and also as 
reflectors and luminous organs (signa) for recognition. Possibly these 
organs of Barathronus indicate the course in development of the ocular 
tracts of Ipnops. 
Most often on deep sea fishes the eyes and the lateral system are both 
well developed, but greater development of the system is likely to be 
attended by reduction in the size of the eye. On sedentary forms which 
mainly depend on tactile developments the eyes are minute, as on species of 
Ceratiid, for instance Dolopichthys allector, Plate XIII.; on others of which 
the main reliance is on sight the eyes are the larger. Free swimming forms 
with excessive tactile developments, again, like Bathypterois, Benthosaurus, 
Dicrolene, and Mixonus have the eyes much smaller, the gize of the organ 
being inversely proportioned to the excess in the tactile organs. There are 
cases in which tactile papille: and the lateral system are both highly devel- 
oped, as on Evetmichthys ocella, for instance, but commonly when one of the 
two is greatly favored the other is more likely to be slighted. 
In response to the demands of bathybial conditions the sensory organs of 
the Lateral Canal System have in many forms become modified from simple 
nerve papille of tactile functions to luminous, flashlight, and, in some at 
least, to electric organs of great complexity. As the system is given special 
treatment below, the reader is referred to it for further discussion. 
In the gills there is evidence of a decrease in the amount of oxygen 
consumed in the depths as compared with that used in the breathing ap- 
paratus of fishes near the surface, and the decrease naturally is accom- 
panied by a lessened amount of activity. The bathybial fishes have 
sinaller oxygenating surfaces, the laminz are reduced in size and in 
many cases the gills are reduced in number. Many of the Halieutoids 
2 
