152 A HISTORY OF FISHES 



of the photophores occur on the lower parts of the body, rarely 

 on the back or upper parts of the sides. Many species have a 

 special organ above or betew the eye, and others possess a very 

 powerful head-lamp covering the greater part of the snout 

 (Fig. 63A). Certain patches may be developed on the head 

 and body, especially at the bases of the fins, which, although 

 luminous, lack the speciahsed structure of the photophores. 

 One or more of these patches may be present on the upper 

 and lower edges of the fleshy part of the tail, where they are 

 described as "stern-chasers." 



The line and bait of the Angler-fishes, representing the much 

 modified first ray of the dorsal fin, has already been described 

 {cf. p. 71). In the deep-sea members of this order [Ceratioidea) 

 the bait is luminous and can be "switched on" at will, serving 

 to attract smaller fishes within reach of the Angler's jaws. In 

 addition to this line and bait certain species possess a much 

 branched tree-like structure below the chin which also has 

 luminous properties and probably acts as a lure (Fig. 31 a). 

 Other deep-sea fishes, in which one or more of the fin-rays 

 may be drawn out to form a fine filament, have these rays 

 tipped with small luminous bulbs. Another remarkable form, 

 known as Ipnops^ has the whole of the upper surface of the 

 flattened head occupied by a pair of large organs lying beneath 

 the transparent superficial bones of the roof of the skull and 

 believed to be luminous (Fig. 63B). It is possible that these 

 take the place of the eyes, which are entirely wanting in this 

 curious and rare fish. 



So far only the light organs of fishes inhabiting the open 

 oceanic waters and descending to a fair depth have been 

 mentioned. These structures may also occur, however, in 

 fishes living more or less close to the shore, having been found 

 in the common Haddock {Gadus aeglifinus) and in one of the 

 Toad-fishes {Porichthys) of the coast of California. The latter 

 fish, known locally as the Midshipman or Singing-fish, has no 

 less than seven hundred photophores on its head and body, 

 each presenting the appearance of a shining white spot. These 

 organs are developed in connection with the complicated 

 system of lateral lines, and many of them are associated with 

 the sense organs. Each consists of four parts: lens, gland, 

 reflector, and pigment, and, as usual, they are more abundant 

 on the lower parts of the fish. 



Although various theories as to the production of lumin- 

 escence ha\'e been ad\'anced, it is now generally agreed that this 



