430 FISHES 



inwards towards the derma. It seems very probable that all 

 true luminous organs, at least all those of the second group, 

 consist of gland-cells and lens-cells derived from the epidermis. 

 The curious organs on the head of Ipnops, however, if they 

 are really luminous organs, show no resemblance in structure 

 to those of any of the three groups which we have distinguished, 

 and must be considered separately. Ipnops has no eyes, and it 

 has been supposed that the flat white pair of organs on the head 

 (Plate XXXI., D.) were really the eyes converted into luminous 

 organs, but according to Moseley there is no evidence of this, as 

 the organs have no resemblance to eyes and are not innervated 

 by the optic nerves, which are entirely wanting. The olfactory 

 organs and nerves are also rudimentary, but the auditory 

 organs are well developed. The luminous organ on each 

 side is situated between the surface of the cartilaginous skull 

 below and a large flat scale above, running longitudinally, in 

 which is a mucous tube apparently belonging to the dermal 

 sensory system. The organ itself consists of a large number 

 of six-sided columns placed vertically on the base of the organ. 

 Each column appears white and glistening at its external sur- 

 face and is separated from its neighbours by black pigment. It 

 consists of 30 to 40 transparent rods resting inferiorly on a layer 

 of pigmented tissue ; beneath it is a single large six-sided pig- 

 ment cell, and the outer extremity of each vertical rod is a six- 

 sided nucleated cell. No distinct nerve connection was traced, 

 but it seemed probable that the nerves passing into the organ 

 from the subjacent fibrous tissue originated from the 5th cranial 

 nerve. Moseley was of opinion that these organs were greatly 

 enlarged representatives of the luminous organs in front of and 

 below the eye in some species of Myctopkum l these organs having 

 united together and occupied the whole of the orbit, while the 

 eye had degenerated. It is difficult, however, to compare any 

 of the elements in the organs of Ipnops with those of the more 

 usual types of light-organ ; it will be noticed that the reflector 

 is entirely absent from the organs of Ipnops. There is one type 

 of organ which has been described in some Sternoptychidae 

 and Stomiatidae which contains cells somewhat similar to the 

 vertical rods of the organ of Ipnops. These organs consist of 

 a spherical inner portion and a conical outer portion like those 

 of Sternoptyx described above, but in the inner portion instead 



