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 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, particularly in the tail (_d_), dorsal (_e_, _f_), ventral (_b_, _c_), and pectoral (_a_), fins, which are principally engaged in swimming. The extent of the travelling surfaces 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, curvi-linear, 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 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,[41] 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.