HOW ANIMALS MOVE 365 



It is also a rule, with some exceptions, that the volun- 

 tary muscles of vertebrates, and all the muscles 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 surround 

 cavities. 135 



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 

 themselves. The last are hollow or circular muscles, 

 inclosing a cavity or space, which they reduce by con- 

 traction. Examples of such are seen in the heart, blood 

 vessels, stomach, iris of the eye, and around the mouth. 

 In the lower invertebrates, the muscular system is a net- 

 work of longitudinal, transverse, and oblique fibers inti- 

 mately blended with the skin, and not divisible into sep- 

 arate muscles. As in the walls of the human stomach, 

 the fibers are usually in distinct layers. This ar- 

 rangement is exhibited by soft-bodied animals, like 

 the sea anemone, the snail, and the earthworm. Four 

 thousand muscles have been counted 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 hardened into a 

 shell or crust have muscles attached to the inside of such 

 a skeleton. Thus, the oyster has a mass of parallel 

 fibers connecting its two valves ; while in the lobster 

 and bee fibers go from ring to ring, both longitudinally 

 and spirally. The muscles of all invertebrates are 



