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If we direct our attention to the water, we encounter a medium less dense than the earth, and considerably more dense than the air. As this element, in virtue of its fluidity, yields readily to external pressure, it follows that a certain relation exists between it and the shape, size, and weight of the animal progressing along or through it. Those animals make the greatest headway which are of the same specific gravity, or are a little heavier, and furnished with extensive surfaces, which, by a dexterous tilting or twisting (for the one implies the other), or by a sudden contraction and expansion, they apply wholly or in part to obtain the maximum of resistance in the one direction, and the minimum of displacement in the other. The change of shape, and the peculiar movements of the swimming surfaces, are rendered necessary by the fact, first pointed out by Sir Isaac Newton, that bodies or animals moving in water and likewise in air experience a sensible resistance, which is greater or less in proportion to the density and tenacity of the fluid and the figure, superficies, and velocity of the animal.

To obtain the degree of resistance and non-resistance necessary for progression in water, Nature, never at fault, has devised some highly ingenious expedients,—the Syringograde animals advancing by alternately sucking up and ejecting the water in which they are immersed—the Medusæ by a rhythmical contraction and dilatation of their mushroom-shaped disk—the Rotifera or wheel-animalcules by a vibratile action of their cilia, which, according to the late Professor Quekett, twist upon their pedicles so as alternately to increase and diminish the extent of surface presented to the water, as

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happens in the feathering of an oar. A very similar plan is adopted by the Pteropoda, found in countless multitudes in the northern seas, which, according to Eschricht, use the wing-like structures situated near the head after the manner of a double paddle, resembling in its general features that at present in use among the Greenlanders. The characteristic movement, however, and that adopted in by far the greater number of instances, is that commonly seen in the fish (figs. 29 and 30).


FIG. 29.-Skeleton of the Perch (Perca fluviatilis). Shows the jointed nature of

the vertebral column, and the facilities afforded for lateral motion, particu-
larly in the tail (d), dorsal (e, J), ventral (b, c), and pectoral (a), fins, which
are principally engaged in swimming. The extent of the travelling sur-
faces required for water greatly exceed those required for land. Compare the
tail and fins of the present figure with the feet of the ox, fig. 18, P. 37.-
(After Dallas.)


FIG. 30.—The Salmon (Salmo salar) swimming leisurely. The body, it will be

observed, is bent in two curves, one occurring towards the head, the other towards the tail. The shape of the salmon is admirably adapted for cleaving the water.—Original.

This, my readers are aware, consists of a lashing, curvilinear, or flail-like movement of the broadly expanded tail, which oscillates from side to side of the body, in some instances with immense speed and power. The muscles in the fish, as has.

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been explained, are for this purpose arranged along the spinal column, and constitute the bulk of the animal, it being a law that when the extremities are wanting, as in the water-snake, or rudimentary, as in the fish, lepidosiren,1 proteus, and axolotl, the muscles of the trunk are largely developed. In such cases the onus of locomotion falls chiefly, if not entirely, upon the tail and lower portion of the body. The operation of this law is well seen in the metamorphosis of the tadpole, the muscles of the trunk and tail becoming modified, and the tail itself disappearing as the limbs of the perfect frog are developed. The same law prevails in certain instances where the anterior extremities are comparatively perfect, but too small for swimming purposes, as in the whale, porpoise, dugong, and manatee, and where both anterior and posterior extremities are present but dwarfed, as in the crocodile, triton, and salamander. The whale, porpoise, dugong, and manatee employ their anterior extremities in balancing and turning, the great organ of locomotion being the tail. The same may be said of the crocodile, triton, and salamander, all of which use their extremities in quite a subordinate capacity as compared with the tail. The peculiar movements of the trunk and tail evoked in swimming are seen to most advantage in the fish, and may now be briefly described.

Swimming of the Fish, Whale, Porpoise, etc.—According to Borelli,2 and all who have written since his time, the fish in swimming causes its tail to vibrate on either side of a given line, very much as a rudder may be made to oscillate by moving its tiller. The line referred to corresponds to the axis of the fish when it is at rest and when its body is straight, and to the path pursued by the fish when it is swimming. It consequently represents the axis of the fish and the axis of

i The lepidosiren is furnished with two tapering flexible stem-like bodies, which depend from the anterior ventral aspect of the animal, the siren having in the same region two pairs of rudimentary limbs furnished with four imperfect toes, while the proteus has anterior extremities armed with three toes each, and a very feeble posterior extremity terminating in two toes.

2 Borelli, “ De motu Animalium,” plate 4, fig. 5, sm. 4to, 2 vols. Romæ, 1680.

motion. According to this theory the tail, when flexed or curved to make what is termed the back or non-effective stroke, is forced away from the imaginary line, its curved, concave, or biting surface being directed outwards. When, on the other hand, the tail is extended to make what is termed the effective or forward stroke, it is urged towards the imaginary line, its convex or non-biting surface being directed inwards (fig. 31).

FIG. 31.-Swimming of the Fish.-(After Borelli.)

When the tail strikes in the direction a i, the head of the fish is said to travel in the direction ch. When the tail strikes in the direction ge, the head is said to travel in the direction cb; these movements, when the tail is urged with sufficient velocity, causing the body of the fish to move in the line dcf. The explanation is apparently a satisfactory one; but a careful analysis of the swimming of the living fish induces me to believe it is incorrect. According to this, the commonly received view, the tail would experience a greater degree of resistance during the back stroke, i.e. when it is flexed and carried away from the axis of motion (d cf) than it would during the forward stroke, or when it is extended and carried towards the axis of motion. This follows, because the concave surface of the tail is applied to the water during what is termed the back or non-effective stroke, and the convex surface during what is termed the forward or effective stroke. This is just the opposite of what actually happens, and led Sir John Lubbock to declare that there was a period in which the action of the tail dragged the fish backwards, which, of course, is erroneous. There is this further difficulty. When the tail of the fish is urged in the direction g e, the head does not move in the direction c b as stated, but in the direction ch, the body of the fish describing the arc of a circle, a ch. This is a matter of observation. If a fish when resting suddenly forces its tail to one side and curves its body, the fish describes a curve in the water corresponding to that described by the body. If the concavity of the curve formed by the body is directed to the right side, the fish swims in a curve towards that side. To this there is no exception, as any one may readily satisfy himself, by watching the movements of gold fish in a vase. Observation and experiment have convinced me that when a fish swims it never throws its body into a single curve, as represented at fig. 31, p. 67, but always into a double or figure-of-8 curve, as shown at fig. 32.1

Fig. 32.-Swimming of the Sturgeon. From Nature. Compare with figs. 18 and 19, pp. 37 and 39; fig. 23, p. 43; and figs. 64 to 73, pp. 139, 141, and 144.-Original.

The double curve is necessary to enable the fish to present a convex or non-biting surface (c) to the water during flexion (the back stroke of authors), when the tail is being forced away from the axis of motion (a b), and a concave or biting surface (s) during extension (the forward or effective stroke of authors), when the tail is being forced with increased energy towards the axis of motion (a b); the resistance occasioned by a concave surface, when compared with a convex one, being in the ratio of two to one. The double or complementary curve into which the fish forces its body when swimming, is necessary to correct the tendency which the head of the fish has to move in the same direction, or to the same side as that

] It is only when a fish is turning that it forces its body into a single curve.

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