MUSCULAR AND SKELETAL SYSTEMS. 



233 



the flight of many flies; the prodigious leaps of fleas, 

 three hundred times their own length ; the enormous 

 masses, twenty times their own weight, dragged by ants, 

 etc. ; and yet the relation of muscle to skeleton, and 

 therefore the mechanism of motion and locomotion, is 

 wholly different from that of vertebrates. In verte- 

 brates we have an internal skeleton and the muscles act- 

 ing on it from the outside ; in the case of arthropods we 

 have an external skeleton and the muscles acting on it 

 from the inside. The whole animal is inclosed in a skele- 

 tal coat of mail. 

 The body is a hol- 

 low, jointed barrel 

 inclosing the vis- 

 cera, and the limbs 

 are hollow pipes 

 filled with muscle. 

 The manner in 

 which this ar- 

 rangement is used 

 for limb motion is 

 shown in the fol- 

 lowing figures: Fig. 149, A and B, represents the joints 

 of a stovepipe beveled a little on the two opposite sides 

 so that when fitted, the one in the other, there is a small 

 vacant space between. Now if the interfitted parts, a a, 

 be riveted together, and strings, m m', within the pipe be 

 attached to the beveled margins, we have a perfect hinge 

 joint, and pulling on one string or the other produces 

 motion in two directions in one plane. Now the mechan- 

 ism for limb motion in all arthropods is like this, except 

 that we have ligaments instead of rivets, and muscle 

 and tendon instead of strings pulled by hand. Fig. 150 

 shows four joints of the limb of a crab or lobster 

 and the manner in which the muscles bend the limbs. 



Fig. 149. — Diagram showing mode of action of 

 muscle and skeleton in an arthropod : a, the 

 joint ; m, the muscle. 



