[42] Borelli, “De motu Animalium,” plate 4, fig. [5], sm. 4to, 2 vols. Romæ, 1680.
[43] It is only when a fish is turning that it forces its body into a single curve.
[44] The Syngnathi, or Pipefishes, swim chiefly by the undulating movement of the dorsal fin.
[45] If the pectoral fins are to be regarded as the homologues of the anterior extremities (which they unquestionably are), it is not surprising that in them the spiral rotatory movements which are traceable in the extremities of quadrupeds, and so fully developed in the wings of bats and birds, should be clearly foreshadowed. “The muscles of the pectoral fins,” remarks Professor Owen, “though, when compared with those of the homologous members in higher vertebrates, they are very small, few, and simple, yet suffice for all the requisite movements of the fins—elevating, depressing, advancing, and again laying them prone and flat, by an oblique stroke, upon the sides of the body. The rays or digits of both pectorals and ventrals (the homologues of the posterior extremities) can be divaricated and approximated, and the intervening webs spread out or folded up.”—Op. cit. vol. i. p. 252.
[46] Vide “Remarks on the Swimming of the Cetaceans,” by Dr. Murie, Proc. Zool. Soc., 1865, pp. 209, 210.
[47] In a few instances the caudal fin of the fish, as has been already stated, is more or less pressed together during the back stroke, the compression and tilting or twisting of the tail taking place synchronously.
[48] The unusual opportunities afforded by this unrivalled collection have enabled me to determine with considerable accuracy the movements of the various land-animals, as well as the motions of the wings and feet of birds, both in and out of the water. I have also studied under the most favourable circumstances the movements of the otter, sea-bear, seal, walrus, porpoise, turtle, triton, crocodile, frog, lepidosiren, proteus, axolotl, and the several orders of fishes.
[49] This is the reverse of what takes place in flying, the anterior or thick margins of the wings being invariably directed upwards.
[50] The air-bladder is wanting in the dermopteri, plagiostomi, and pleuronectidæ.—Owen, op. cit. p. 255.
[51] The frog in swimming leisurely frequently causes its extremities to move diagonally and alternately. When, however, pursued and alarmed, it folds its fore legs, and causes its hind ones to move simultaneously and with great vigour by a series of sudden jerks, similar to those made by man when swimming on his back.