HOW ANIMALS MOVE. 157 



which are coarsest in Fishes (most of all in the Rays), and 

 finest in Birds, are bound into bundles by connective tis- 

 sue; and the muscles thus made up are arranged in layers 

 around the skeleton. Sometimes their extremities are at- 

 tached to the bones (or rather to i\\% periosteum) directly ; 

 but generally by means of white inelastic cords, called 

 tendons. In Fishes, the chief masses of muscle are dis- 

 posed along the sides of the body, apparently in longitu- 

 dinal bands, reaching from head to tail, but really in a 

 series of vertical flakes, one for each vertebra. In propor- 

 tion as limbs are developed, we find the muscles concen- 

 trated about the shoulders and hips, as in quadrupeds. 

 The bones of the limbs are used as levers in locomotion, 

 the fulcrum being the end of a bone with which the mov- 

 ing one is articulated. Thus, in raising the arm, the hu- 

 merus is a lever working upon the scapula as a fulcrum. 

 The most important muscles are called extensors aud flex- 

 ors. The latter are such as bring a bone into an angle 

 with its fulcrum as in bending the arm while the for- 

 mer straighten the limb. Abductws draw a limb away 

 from the middle line of the body, or a finger or toe away 

 from the axis of the limb, while adductors bring them back. 



2. Locomotion. All animals have the power of vol- 

 untary motion, and all, at one time or another, have the 

 means of moving themselves from place to place. Some 

 are free in the embryo-life, and fixed when adult, as the 

 Sponge, Coral, Crinoid, and Oyster. There may be no 

 regular well-defined means of progression, as in the Amoe- 

 ba, which extemporizes arms to creep over the surface; 

 or movement may be accomplished by the contraction of 

 the whole body, as in the Jelly-fish, which, pulsating about 

 fifteen times in a minute, propels itself through the water. 

 So the "Worms and Snakes swim by the undulations of the 

 body. 



But, as a rule, animals are provided with special organs 



