32 ANIMAL LIFE 



powerful down strokes that give effectiveness to its 

 flight 



But whilst we may in this way gain some acquaint- 

 ance with animal movement by the analogy of a 

 boat, there are features in the build and movements 

 of animals not easily paralleled. A boat is a stiff 

 unyielding structure, whilst an animal is provided with 

 mobile muscular walls, which elongate and become 

 thinner, or contract and thicken. Thus we may see 

 worms, leeches, and caterpillars extend their bodies 

 and, laying hold of some projection, draw themselves 

 up to it, measuring in this way by looping movements 

 the ground over which they travel, and so earning the 

 sobriquet geometers ; or, by more rapidly repeating 

 the movement, the separate events of tension and 

 extension become merged into a continuous gliding. 

 Thus the snake uses the special scales arranged down 

 its belly as projections for performing those rippling 

 movements by which it almost swims over the ground, 

 and in so doing helps the forward movement by 

 lateral undulations of the tail. 



A method commonly employed amongst lower 

 animals, and one that has no analogy in the working 

 of a boat, consists in the expulsion of water from their 

 bodies, and of a consequent rebound in the opposite 

 direction. By this means jelly-fish swim in a series 

 of jerks, which are due to the hollow bell or bell-clapper 

 alternately expanding and taking a draught of water, 

 then contracting and expelling it. 



In a similar manner cuttlefish, octopus, and the 



