FORM AND LOCOMOTION 23 



give a good idea of the manner in which the body is undulated, 

 and show how the flexure may be traced backwards from the 

 head towards the tail (Fig. 9). 



The actual forward thrust is effected by the pressure of the 

 fish's body against the surrounding water, aided to a certain 

 extent by the tail-fin stroke, due to the action of the muscles. 

 In order to understand the action of the body movements on 

 the surrounding medium it will be convenient to study an 

 elongated type offish such as the Eel (Anguilla), and compare 

 its locomotor methods with those of the Mackerel. The move- 

 ment is again initiated by the contraction of the first few 

 myomeres on one side. The anterior part of the body is thus 

 thrown into a curve, and this curve is passed backwards in a 

 series of waves by the alternate contractions and expansions of 

 the serial muscle segments. The movement is mechanically 

 the same as that of a long rope held at one end and given a 

 smart jerk with the hand at right angles to its axis. This results 

 in a wave passing down the rope, the curves gradually de- 

 creasing in size and eventually dying out because the initial 

 action of the hand was the sole agent of propulsion ; in the living 

 fish each successive muscle segment gives an added impetus to 

 the wave, and, as soon as the first wave has started backwards, 

 a second follows, but on the opposite side, and so on. Here 

 the forward thrust is attained almost entirely by the pressure 

 of the fish's body against the water contained in the spaces 

 betv/een the curves. With this elongate form the fish gains 

 much greater pressure areas from its sides than does the 

 Mackerel, but, at the same time, it naturally loses the terminal 

 effect of the tail-fin. Indeed, we find that in all eel-like fishes 

 the caudal fin is either very much reduced or wanting altogether, 

 a good example of the inevitable disappearance of a useless 

 organ. In most Eels the anterior part of the body is cylindrical 

 in cross-section, whereas the hinder part is distinctly compressed ; 

 this feature has a mechanical advantage, since a blade-like 

 structure which presents its surface more effectively to the 

 water naturally provides a greater amount of thrust than a 

 rounded one. The elongate Ribbon-fishes {Regalecus) , and other 

 fishes with long bodies greatly flattened from side to side (Fig. 

 8d), undulate them into curves which are even more ample 

 than those of the Eel, the extreme ribbon shape making this 

 excessive bending comparatively easy to perform. It is of 

 interest to note here that fishes with the body rounded can 

 move over soHd surfaces out of water by applying the same 



