VENOM, ELECTRICITY, LIGHT, AND SOUND 15! 



ship after capture, and continued to emit a luminous glow until 

 its death three hours later. 



Among the Bony Fishes, the members of the great group of 

 Wide-mouths {Stomiatoidea) , inhabitants of the open oceans, 

 many of them descending to considerable depths, possess 

 phosphorescent organs which may be of a much more elaborate 

 structure. There are typically two rows of organs, or photo- 

 phores, on either side of the fish, one on the belly and another 

 parallel to it and near the lower edge of the side, but in some 

 species additional series may be developed above these (Fig. 91). 

 The rows generally extend continuously from behind the head 

 to the tail, but may be interrupted in some species and confined 

 to certain regions of the body and tail. The arrangement is 

 metameric, that is to say, there is one set of four organs to 

 each muscle segment or section of the vertebral column. The 

 organs may be of a comparatively simple structure, consisting 

 of little more than a group of gland-cells, not very different 

 from the ordinary groups of cells in the epidermal layer of the 

 skin from which the photophores have been derived. Some 

 of them, however, may be more elaborate in structure, con- 

 sisting of a lens set in the opening of a cup which is sunk in the 

 skin, the walls of the cup being made up of the gland-cells which 

 manufacture the light-giving substance: the walls may be lined 

 with black pigment to form a reflector similar to that used in 

 a bulFs-eye lantern, and the outer skin partially projects over 

 the surface of the lens and functions like the diaphragm or 

 stop of a camera. In addition to the body photophores, there 

 are others on the head and jaws, including a large and some- 

 times complicated organ below or just behind each eye. This 

 organ has a backing of black pigment, and is freely movable, 

 being rolled inwards when not required. A still more curious 

 organ is found in a rare Berycoid fish from the Indian Ocean 

 known as Anomalops. This fish has a large phosphorescent 

 organ below the eye, placed on a movable flap, so that when 

 light is not wanted it can be turned inwards and received into 

 a cavity underneath the eye (Fig. 63c). 



In the Myctophids or Lantern - fishes {Myctophidae) the 

 photophores are fewer in number, but are larger and brighter, 

 having the appearance of glistening jewels or small mother-of- 

 pearl buttons. Instead of being set out in rows running from 

 the head to the tail, they have a complicated arrangement of 

 short rows and groups, which is, nevertheless, perfectly sym- 

 metrical (Fig. 63A). As in the Wide-mouths, the great majority 



