412 FISHES. 



skate, or from side to side like the plaice and sole, usually 

 live more or less on the bottom ; eel-like forms often wallow 

 in the mud, or creep in and out of crevices ; globe-fishes, 

 like Diodon and Tetrodon, often float passively. There are 

 manystrange fishes, such as the se3.-\iorsQs{e.g.,JIippocampus), 

 which play among- the sea-weeds in warm seas. Some of 

 the deep-sea fishes are very quaint. 



The colours of Fishes are often very bright. They de- 

 pend partly on pigments in the cells of the skin, partly on 

 the physical structure of the scales. In many cases the 

 colours of the male are brighter than those of his mate, 

 witness the gemmeous dragonet {Callionymus lyrd) and the 

 stickleback {Gasterosteus), and this is especially true at the 

 breeding season. The colours of many fishes change not 

 a little in harmony with their surroundings. In the plaice 

 and some others the change is rapid. Surrounding colour 

 affects the eye, the influence passes from eye to brain, 

 and from the brain down the sympathetic nervous system, 

 thence by peripheral nerves to the skin, where the size 

 of the amceba-like pigment-cells is affected. In shallow 

 and clear water this power of colour-change may be of much 

 protective value, but it seems likely that this has been ex- 

 aggerated. In soles, flounders, plaice, and other " flat-fish," 

 which in adult life lie or swim on one side, only the exposed 

 surface is pigmented, the hidden side being white. It has 

 been shown by J. T. Cunningham that an artificial illumina- 

 tion of the lower side induces the development of pigment, 

 therefore we may say that the normal whiteness of this side is 

 somehow due to the absence of light stimulus. An appre- 

 ciation of the protective value of this difference of colouring 

 demands careful attention to the habits and habitat of these 

 fishes, to the nature of the light in which they live, and to 

 the enemies which are likely to attack them. 



The food of Fishes is very diverse — from Protozoa to 

 Cetaceans. Sharks and many others are voraciously carni- 

 vorous, many engulf worms, crustaceans, insects, molluscs, 

 or other fishes, others browse on sea-weeds, or swallow mud 

 for the sake of the living and dead organisms which it 

 contains. Their appetite is often enormous, and cases are 

 known {e.;;., Chiasmodon niger), where a fish has swallowed 

 another larger than its own normal size. 



