66 



ANIMAL LOCOMOTION. 



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,^ pro tens, 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 tad- 

 IDole, 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 sub- 

 ordinate 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, TFhale, Porpoise, etc. — ^According to 

 Borelli,^ 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 



1 The lepidosiren is furnished with two tapering flexible stem-like bodies, 

 ^hich depend from the anterior ventral aspect of the animal, the siren having 

 in the same region two pairs of rudimentary limbs furnished with four imper- 

 fect 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. Romae, 

 1G80. 



