12 THE COMMERCIAL SEA FISHES OF 



coloured and endowed with sarcodic movements, capable of 

 marked changes of form under special influences, so as to 

 present the shape of extended dendritic surfaces, or minute 

 spherical masses, through which the pigment is distributed. 

 The changes of colour due to thin plates are of course 

 exceedingly variable, the tints following each other with 

 great rapidity according to the angle at which we view 

 them. Such is seen among insects, Crustacea, and some 

 families of fishes, as those of the dolphin, Coryphczna ; while 

 the second class of colours, those due to the movements of 

 the anatomical elements, are directly connected with the 

 impression of colour received by the eye, and brought about 

 by the reflex action of the nervous system (Agassiz). 



Due to certain causes, fish may assume colours which 

 are not natural to the species, as black, when it is termed 

 melanism, red, known as erythrism, yellow or leuccethiopism, 

 these varieties being most common in fresh-water forms. 

 There is also a piebald or white form, albinoism, very com- 

 mon in flat fishes, which is probably more or less hereditary, 

 and may have been originally due among pleuronectoids to 

 a right and a left variety breeding together. 



Another external phenomenon sometimes observed 

 among marine fishes is a luminous appearance, perceived 

 either when the creature was alive or soon after death. The 

 shark is one of the former which has the reputation of being 

 luminous, the light being reputed to proceed from its 

 abdominal surface. Shoals of fish frequently emit flashes 

 of light, visible at great distances, and often due to the 

 collision of the fish with minute light-producing animals. 

 We also find a general luminosity which exists for some 

 time in a dead fish, commencing a short time subsequent 

 to death and continuing until decomposition sets in. In 

 such forms as decompose more rapidly, luminosity is most 



