LOCOMOTION OF ANIMALS IN FLUIDS. 187 



the straight position, during which movement it strikes the water obliquely ; 

 next passing beyond the middle line, and curving slightly to the opposite side, 

 it again repeats a sharp backward stroke, striking the water obliquely, across 

 the direction of the former stroke, and once more passing the middle line, it 

 repeats the former stroke, and so on in succession, on one side and the other. 

 The reaction of the water takes place, of course, obliquely forwards, in the 

 opposite direction to each backward stroke of the tail, and so tends to urge 

 the centre of gravity of the fish in oblique zigzag lines forward through the 

 water ; but in rapid swimming, these two oblique forces are combined into a 

 median resultant force, which impels the fish directly and swiftly forwards. 

 Even then, however, a slight vibrating motion of the head is sometimes visible, 

 the evidence of the double origin of its forward impulse. The question has 

 often been asked, why is it that the resistance of the water to the forward 

 curving of the tail, previous to its making the backward stroke, and in pass- 

 ing beyond the middle line after one backward stroke, previous to its perform- 

 ing another, does not check the movement of the fish forward, by counter- 

 balancing the impulse arising from the backward movements of the tail ? The 

 reply to this question appears to be twofold : first, the backward stroke is de- 

 livered not only with greater force, but with greater velocity than the forward 

 curving of the tail ; and as time is always an element in measuring the effects 

 of force, the backward stroke is superior in practical effect ; secondly, the re- 

 sistance to the forward movement of the fish in the water, is reduced, as we 

 have seen, to the smallest possible amount, first, by the form of the body, and, 

 secondly, by its slipperiness, w r hich prevents friction, whereas to any back- 

 ward movement of the fish in the water, the edges of the scales, elevated 

 slightly from each other by the curving forwards of the tail, take powerful 

 hold of the surrounding fluid, and so offer great resistance to any sliding back- 

 wards of the fish. Whoever has hooked a fish accidentally by the tail, knows 

 how much more difficult it is to lift or drag it through the water, than if it 

 had been caught by its seizing the bait in the ordinary way ; a fact which 

 proves the resistance offered by the edges of the scales. 



In describing the act of swimming in the fish, we must not omit to mention 

 the existence, in most species, of the air-bladder, or swimming-bladder. This 

 is an elongated receptacle, containing gases secreted by its lining membrane ; 

 it varies in form and size, and is situated in the upper part of the abdominal 

 cavity, immediately under the vertebral column ; sometimes it is completely 

 closed, but at other times it communicates by a narrow, elongated neck, with 

 the throat, pharynx, or some part of the alimentary canal, of the fish. The 

 air-bladders of fishes will be referred to again in the chapter on Respiration ; 

 but, in connection with our present subject, it is obvious that its presence, or 

 absence, and its relative state of distension with gaseous matter, must mate- 

 rially affect the specific gravity or buoyancy of a fish. It is supposed that the 

 fish has some power of regulating the quantity of gas contained within it, and 

 so of adjusting its own specific gravity, to enable it to rise or fall in the water ; 

 but no such faculty has been actually proved to exist, and it seems somewhat 

 improbable that the vito-chemical function of such an organ, which is the 

 homologue of the lung, should be subjected directly to the will. It is notice- 

 able also, that the air-bladder appears to be very capriciously distributed 

 amongst the fish tribe, for it is wanting, not only in certain genera amongst 

 certain orders, but also in certain species of a genus, other species of which 

 possess it. Thus there is no air-bladder in the common mackerel, though one 

 species of the same genus (Scomber) possesses it. It is, however, small in 

 fishes which bury themselves in the mud, or live habitually at the bottom of 

 the water ; in the common perch, it is closed. Even when fully distended 

 with air, almost to bursting, as I have demonstrated on various dead fish, it 

 fails to render the entire body buoyant in water, for this nevertheless sinks to 

 the bottom. Mechanically considered, it can therefore only reduce the specific 

 gravity in a certain measure, and so economize the muscular effort which is 

 still necessary to enable the fish to ascend in that fluid ; whilst the absorption 

 of its gaseous contents will, on the contrary, increase the rapidity of the de- 

 scent of the fish by its gravity merely. A possible explanation of its use may 

 be, that it serves to render the ordinary attitude of a fish, with its back up- 



