508 PISCES— FISHES. 



the residue of yolk, which, by its buoyancy, causes the young fish to be 

 suspended in the water back downwaids. 



General Notes on Functions, Habits, and 

 Life Histories of Fishes. 



Movement. — A fish may well compare with a bird in its mastery of 

 the medium in which it lives. Thus a salmon travels at the rate of 

 about eight yards in a second, or over sixteen miles an hour. The 

 motion depends mainly on the powerful 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. The large pectoral fins of the flying-fish [Dactylopterus and 

 Exoccetus) are used rather as parachutes than as wings, during the long 

 skimming leaps. They vibrate strongly but passively against the air, or 

 when the tail strikes the water. Indeed, their movements have some 

 directive, but no locomotor significance. 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. 



Shape in relation to habit. — The characteristic form of the 

 body, as seen in herring or trout, is an elongated laterally compressed 

 spindle, thinning oft' behind like a wedge. In most cases the trunk 

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

 torpedo-like 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 forms, such as the sea-horses (e.g. 

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

 of the deep-sea fishes have also very quaint shapes. 



Colour. — The colours of Fishes are often very bright. They depend 

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

 structure of the scales. The common silvery colour is due to small 

 crystals of guanin in the scales. In many cases the colours of the male 

 are brighter than those of his mate, as in the gemmeous dragonet 

 (Callionymus lyra) and the stickleback (Gasterosteus), and this is 

 especially true at the breeding season. The colours of many fishes 

 change 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 

 distribution of the pigment granules in the cells is altered. In shallow 

 and clear water this power of colour-change may be protective, but an 

 appreciation of the protective value of colouring demands careful 



