72 



ANIMAL LOCOMOTION. 



portion as the curves into which the body is thrown in swim- 

 ming are diminished, the degree of rotation at the tail or in 

 the fins is augmented, some fishes, as the mackerel, using the 

 tail very much after the manner of a screw in a steam-ship. 

 The fish may thus be said to drill the water in two directions, 

 viz. from behind forwards by a twisting or screwing of the 

 body on its long axis, and from side to side by causing its 

 anterior and posterior portions to assume opposite curves. 

 The pectoral and other fins are also thrown into curves when 

 in action, the movement, as in the body itself, travelling in 

 spiral waves ; and it is worthy of remark that the wing of 

 the insect, bat, and bird obeys similar impulses, the pinion, as 

 I shall show presently, being essentially a spiral organ. 



The twisting of the pectoral fins is well seen in the com- 

 mon perch {Perca fluviatilis), and still better in the 15-spined 

 Stickleback {Gasterosteus spinosus)^ which latter frequently 

 pi ogresses by their aid alone.^ In the stickleback, the pec- 

 toral fins are so delicate, and are plied with such vigour, that 

 the eye is apt to overlook them, particularly when in motion. 

 The action of the fins can be reversed at pleasure, so that it 

 is by no means an unusual thing to see the stickleback pro- 

 gressing tail first. The fins are rotated or twisted, and their 

 free margins lashed about by spiral movements which closely 

 resemble those by which the wings of insects are propelled.'^ 

 The rotating of the fish upon its long axis is seen to advan- 

 tage in the shark and sturgeon, the former of which requires 

 to turn on its side before it can seize its prey, — and likewise 



1 The Syngnathi, or Pipefishes, swim chiefly by the undulating movement 

 of the dorsal fin. 



2 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 Pro- 

 fessor Owen, " though, w^hen compared with those of the homologous mem- 

 bers 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." — Ojp. cit. vol. i. p. 252. 



