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 a very 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. Thisxoncave lining 

 directed forward and upward, and to some extent toward the side, probably 

 in part serves as a reflector, but it contains a small spot of black pigment, 

 a httle 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 {sigim) 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 

 Ceratiida?, for instance Dohpichthjs allecior, 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 size of the organ 

 being inversely proportioned to the excess in the tactile organs. There are 

 cases in which tactile papillae and the lateral system are both highly devel- 

 oped, as on Erelmiclithjs ocella, for instance, but couimonly 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 papillae 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 

 smaller oxygenating surf;ices, the lamina^ are reduced in size and in 

 many cases the gills are reduced in number. Many of the Halicutoids 



