298 THE MECHANICAL FUNCTIONS. 



organ is kept folded under the abdomen, where 

 it is concealed in a groove. The pieces of 

 Tvhich it is composed are articulated together in 

 such a manner as to admit of their being rapidly- 

 unbent by the action of its muscles, the whole 

 mechanism conspiring to produce the effect of a 

 powerful spring, by which the body is propelled 

 forwards to a considerable distance. In some 

 species, this flexible tail has a flattened form, 

 for the purpose of enabling the insect to leap 

 from the surface of water, an action which it 

 performs with apparently as much ease as if it 

 sprung from a solid resisting plane. 



The Lepisma leaps by means of moveable 

 appendages, placed in a double row along the 

 under side of the body, and acting like springs. 

 There are eight pair of these members, cor- 

 responding in situation and structure to the 

 false feet of the Crustacea, and, like them, ter- 

 minating in jointed filaments. 



The Julus and the Scolopendra, which com- 

 pose the family of the Myriapoda, so called from 

 the immense number of their feet, undergo, to a 

 certain extent, a kind of metamorphosis in the 

 progress of their developement. When first 

 hatched they have often no feet whatever, and 

 resemble the simpler kinds of worms. Legs at 

 length make their appearance ; but they arise in 

 succession, and it is not until the later periods 

 of their growth that these animals acquire their 



