HOW ANIMALS MOVE. 
157 
which are coarsest in Fishes (most of all in the Kays), 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 the 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 bod} T , 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 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. Abductors 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 
