HABITS OF FISHES 627 



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. 



Shape in relation to habit.— 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. This torpedo-like form is 

 well adapted for rapid progression. 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. 



Colour. — The colours of fishes are often very bright. They 

 depend partly on the presence of pigment cells in the skin, partly on 

 the physical structure of the scales. The common silvery colour is 

 due to small crystals of guanin in the skin. In many cases the colours 

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

 ^dragonet {Callionynius 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 

 attention to the habits and habitat of the fishes, to the nature of the 

 light in which they live, and to the enemies which are likely to attack 

 them. 



Food. — The food of Fishes is very diverse — from Protozoa to 

 Cetaceans. Sharks and many others are voraciously carnivorous ; 

 many engulf worms, crustaceans, insects, molluscs, or other fishes ; 

 others browse on seaweeds, 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.g. Chiasmodon niger) where a fish 

 has swallowed another larger than its own normal size. Many fishes 

 follow their food by sight ; many by a diffuse sensitiveness, to which it 

 is difficult to give a name ; a few, it would seem, by a localised sense 

 of smell. It is important to realise that fishes depend very largely on 

 small crustaceans, and these again on unicellular plants and animals. 

 Just as we may say that all flesh is grass, so we may say that all fish is 

 Diatom. 



Senses, etc. — Fishes do not seem to have much sense of taste or of 



