156 
COMPARATIVE ZOOLOGY. 
of the Lobster, Spider, and Insect tribes, are striated; while 
the involuntary muscles of Vertebrates, and all the muscles 
of Radiates, Worms, and Mollusks, are smooth. All mus¬ 
cles attached to internal bones, or to a jointed external 
skeleton, are striated. The voluntary muscles of Verte¬ 
brates are generally solid, and the involuntary hollow. 81 
This leads to another classification of muscles: into 
those which are attached to solid parts within the body; 
those which are attached to the skin or its modifications; 
and those having no attachments, being complete in them¬ 
selves. The last are hollow or circular muscles, enclosing 
a cavity or space, which they reduce by contraction. Ex¬ 
amples of such are seen in the heart, blood-vessels, stom¬ 
ach, iris of the eye, and around the mouth. In the lower 
Invertebrates, the muscular system is a net-work of longi¬ 
tudinal, transverse, and oblique fibres intimately blended 
with the skin, and not divisible into separate muscles. As 
in the walls of the human stomach, the fibres are usually 
in three distinct layers. This arrangement is exhibited by 
soft-bodied animals, like the Sea-anemone, the Snail, and 
the Earth-worm. Four thousand muscles have been count¬ 
ed 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 otf 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 
inside of such a skeleton. Thus, the Oyster has a mass 
of parallel fibres connecting its two valves; while in the 
Lobster and Bee fibres go from ring to ring, both longi¬ 
tudinally 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, 
