154 COMPARATIVE ZOOLOGY. 
in a Caterpillar. There are also “skin-muscles” in the 
higher animals, as those by which the Horse produces a 
twitching of the skin to shake off Insects, and those by 
which the hairs of the head and the feathers of Birds are 
made to stand on end. Invertebrates, whose skin is hard- 
ened into a shell or crust, have muscles attached to the in- 
side of such a skeleton. Thus, the Oyster has a mass of 
parallel fibres connecting its two valves; while in the Lob- 
ster and Bee, fibres go from ring to ring, both longitudi- 
nally and spirally. The muscles of all Invertebrates are 
straight parallel fibres, not in bundles, but distinct, and 
usually flat, thin, and soft. 
The great majority of the muscles of Vertebrates are 
attached to the bones, and such are voluntary. The fibres, 
which are coarsest in Fishes (most of all in the Rays), and 
finest in Birds, are bound into bundles by a web-like 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 periostewm) 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 and jlex- 
ors. The former are such as pass over the back of a joint 
to extend the bone beyond it; while the flexors lie in front 
of the joint to bring the same bone into an angle with its 
fulerum—as in bending the arm. 
