FORM AND MOVEMENT COLOUR. 517 



GENERAL NOTES ON FISHES. 

 Form and Movement. 



A fish may well compare with a bird in its mastery of the 

 medium in which it lives. Thus a salmon is said to travel 

 at the rate of about eight yards in a second, or over sixteen 

 miles an hour. The motion depends mainly on the power- 

 ful muscles which produce the lateral strokes of the tail and 

 posterior part of the body. It may be roughly compared 

 to the motion of a boat propelled by an oar from the 

 stern. So energetic are the strokes that a fish is often able 

 to leap from the water to a considerable height. In some 

 cases undulating movements of the unpaired fins, and even 

 the rapid backward outrush of water from under the gill 

 cover, seem to help in movement. The paired fins are 

 chiefly used in ascending and descending, in steering and 

 balancing, and some observers state that the pectoral fins of 

 the flying fish are distinctly moved during the long skimming 

 leaps. In a few cases, as in the climbing perch, and in the 

 strange Periophthalmus, which clambers on the mangrove 

 roots, the fore fins and tail are used in scrambling. 



The characteristic form of the body, as seen in herring or 

 trout, is an elongated laterally compressed spindle, thinning 

 off behind like a wedge. In most cases the trunk passes 

 quite gradually into head and tail. It is evident that this 

 form is well adapted for rapid progression through the water. 

 Flat fishes, whether flattened from above downwards, like 

 the 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 many strange fishes, such as the sea horses (e.g., 

 Hippocampus), which play among the sea-weeds in warm 

 seas. Some of the deep-sea fishes have very quaint shapes. 



Colour. 



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. The common silvery 

 colour is often due to small crystals on the scales. In many 

 cases the colours of the male are brighter than those of his 



