CHAPTER XII 



LOCOMOTION" IN WATER 



Summary. — Different types of locomotion in water — Method of photo- 

 graphing aquatic animals — Jelly fish : Comatulre — Locomotion 

 by means of undulatory and lateral movements; the eel--best 

 arrangement for studying its movements — Locomotion by means 

 of undulatory and vertical movements; the skate — special 

 arrangement for studying its vertical undulations from different 

 points of view — Undulatory movements of the skate as seen from 

 the side: ditto as seen from in front — Tne sea-horse: the fresh- 

 water tortoise — Slow movements of star-fish — Locomotion of small 

 marine animals. 



Different Types of Locomotion in Water. — Terrestrial 

 animals make use of the ground as a fulcrum or fixed 

 point of support, and their various kinds of locomotion 

 depend on the following mechanism : A more or less 

 sudden effort on the part of the limbs tends to repel 

 the ground in one direction, and the body in the other. 

 Now, since the ground offers almost absolute resistance, 

 the whole effect of the muscular effort is expended on 

 the body of the animal. 



The locomotion of aquatic animals is quite different, 

 with them the fulcrum, or point of support, is a dis- 

 placeable liquid, and hence more or less of the muscular 

 energy is expended on useless work. 



The various kinds of propellers which men think 

 they have invented for the purposes of navigation, such 

 as sails, oars, and sculls, are represented in the highest 

 degree of perfection in the locomotor organs of aquatic 



