PEOGRESSION ON AND IN THE WATER. 



If we direct our attention to the water, we encounter a 

 medium less dense than the earth, and considerably more 

 dense than the air. As this element, in virtue of its fluidity, 

 yields readily to external pressure, it follows that a certain 

 relation exists between it and the shape, size, and weight of 

 the animal progressing along or through it. Those animals 

 make the greatest headway which are of the same specific 

 gravity, or are a little heavier, and furnished with extensive 

 surfaces, which, by a dexterous tilting or twisting (for the one 

 implies the other), or by a sudden contraction and expansion, 

 they apply wholly or in part to obtain the maximum of re- 

 sistance in the one direction, and the minimum of displace- 

 ment in the other. The change of shape, and the peculiar 

 movements of the swimming surfaces, are rendered necessary 

 by the fact, first pointed out by Sir Isaac Newton, that bodies 

 or animals moving in water and likewise in air experience a 

 sensible resistance, which is greater or less in proportion to 

 the density and tenacity of the fluid and the figure, superficies, 

 and velocity of the animal. 



• To obtain the degree of resistance and non-resistance neces- 

 sary 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 

 Witer in which they are immersed — the Medusae by a rhyth- 

 mical 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 



