310 THE MECHANICAL FUNCTIONS. 



animals. Some of them are destitute of feet, or 

 other external instruments of motion, swimming 

 only by means of the alternate inflexions of the 

 body from side to side, in the same manner as 

 the Nais, and the Leech. Sometimes these 

 actions are performed by abrupt strokes, giving 

 rise to an irregular zig-zag course : this is the 

 case with the larva of the gnat, and with many 

 others which have no feet. In the structure of 

 the larva of the Libellula, or dragon-fly, a sin- 

 gular artifice has been resorted to for giving an 

 impulse to the body, without the help of external 

 members. It is that of the alternate absorption 

 of water into a cavity in the hinder part of the 

 body, and its sudden ejection from that cavity, so 

 that the animal is impelled in a contrary direc- 

 tion, upon the same principle that a rocket rises 

 in the air by the reaction of that fluid. It has 

 at various times been proposed to apply the 

 power of steam to the production of an effect 

 exactly similar to that of which Nature here pre- 

 sents us with so perfect an example, for the pur- 

 pose of propelling ships, instead of the ordinary 

 mode of steam navigation. 



Some larvae, such as that of the Stratiomys, 

 collect a bubble of air, which they retain within 

 a tuft of hair at the extremity of the tail, evi- 

 dently with a view of diminishing the specific 

 gravity of the body, and thus giving greater 

 efficacy to the muscular actions which they 



