Ipnopidæ.—In the Ipnopidæ the head is depressed above and the two eyes are flattened and widened so as to occupy most of its upper surface. These structures were at first supposed to be luminous organs, but Professor Moseley has shown them to be eyes. "They show a flattened cornea extending along the median line of the snout, with a large retina composed of peculiar rods which form a complicated apparatus destined undoubtedly to produce an image and to receive especial luminous rays." The single species, Ipnops murrayi, is black in color and found at the depth of 2½ miles in various seas.
Fig. 90.—Ipnops murrayi Günther.
The existence of well-developed eyes among fishes destined to live in the dark abysses of the ocean seems at first contradictory, but we must remember that these singular forms are descendants of immigrants from the shore and from the surface. "In some cases the eyes have not been specially modified, but in others there have been modifications of a luminous mucous membrane leading on the one hand to phosphorescent organs more or less specialized, or on the other to such remarkable structures as the eyes of Ipnops, intermediate between true eyes and phosphorescent plates. In fishes which cannot see, and which retain for their guidance only the general sensibility of the integuments and the lateral line, these parts soon acquire a very great delicacy. The same is the case with tactile organs (as in Bathypterois and Benthosaurus), and experiments show that barbels may become organs of touch adapted to aquatic life, sensitive to the faintest movements or the slightest displacement, with power to give the blinded fishes full cognizance of the medium in which they live."
Rondeletiidæ.—The Rondeletiidæ are naked black fishes with small eyes, without adipose fin and without luminous spots, taken at great depths in the Atlantic. The relationship of these fishes is wholly uncertain.
Fig. 91.—Cetomimus gillii Goode & Bean. Gulf Stream.
The Cetomimidæ are near allies of the Rondeletiidæ, having the mouth excessively large, with the peculiar form seen in the right whales, which these little fishes curiously resemble.
Fig. 92.—Headlight Fish, Diaphus lucidus Goode & Bean. Gulf Stream.