526 THE BIOLOGY OF MARINE ANIMALS 



tail band, beginning at the margin (minnow Phoxinus). An explanation 

 advanced to account for these facts is that dispersing nerve fibres are 

 present as well as concentrating nerve fibres, and that the injury initiates a 

 stream of impulses in the dispersing fibres. Fading is due to diminution 

 of impulses in these nerve fibres, which are stimulated anew by a fresh cut. 

 Alternatively, the injury may release the melanophores from the control 

 of concentrating fibres and allow an inherent dispersing tendency in the 

 melanophores to operate (29a, 55). 



Other observations based on fin cutting and electrical stimulation have 

 been explained on the basis of concentrating and dispersing nerve fibres. 

 During the regeneration period which follows tail injury in Fundulus, the 

 melanophores show differential behaviour. On black or white backgrounds 

 some of the chromatic cells are capable of complete concentration but not 

 full expansion; some respond by full pigment dispersion but incomplete 

 concentration; some react normally by complete dispersion and concentra- 

 tion; others fail to show any response. These results are explained on the 

 basis of differences in the regeneration of individual nerve fibres : some 

 melanophores have received one or other kind of nerve fibre but not both ; 

 others have regained both dispersing and concentrating fibres ; while other 

 cells still lack innervation (1, 53, 70). 



Additional evidence for the innervation of melanophores has been 

 sought in the effects of drugs which simulate the action of autonomic 

 fibres. Adrenaline and ephedrine cause the melanophores to contract; after 

 treatment with ergotoxine (which blocks the action of adrenaline), the 

 melanophores expand to adrenaline. Acetylcholine, pilocarpine and eserine 

 induce melanophore expansion. Certain differences exist in the behaviour 

 of the species tested with these drugs (Fundulus, Salmo, Pleuronectes, etc.). 

 The action of acetylcholine is linked with the activity of dispersing nerve 

 fibres, and that of adrenaline with concentrating nerve fibres, which are 

 regarded as adrenergic. Teleosts, it may be noted, also possess suprarenal 

 glands located in the anterior kidney region, and adrenaline secretion from 

 these structures possibly contributes to the temporary pallor accompany- 

 ing handling or excitement (53, 59, 63). 



Apart from adrenaline the chromatophores of teleosts are subject to 

 some degree of hormonal control from the pituitary gland. The inter- 

 mediate lobe produces intermedine or B-substance which affects the degree 

 of dispersion of the melanophores. In more primitive teleosts such as 

 Anguilla, posterior pituitary extracts bring about darkening and melanin 

 dispersion; injection of such extracts in many other species — Girella, 

 Gobius, Pleuronectes — has resulted in melanophore contraction. Injection 

 of pituitary extracts is without effect on the melanophores of a normal 

 specimen of Fundulus, but results in darkening of a denervated area of the 

 skin; there is thus some evidence that the melanophores are sensitive to 

 intermedine in this species (14, 54, 70, 71, 74). 



In Fundulus and in many other teleosts the dominant mechanism of 

 chromatophore control is nervous, and this has to a large degree sup- 



