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GENERAL CONCEPTS 



Figure 5.9. The muscles and bones of the forearm, showing the antagonistic arrange- 

 ment of the biceps and triceps muscles. 



right angles to those in the endoderm; contraction of one or the other 

 decreases either the length or the diameter of the body. Flatworms 

 typically have muscle fibers oriented in three different planes, but round- 

 worms have only longitudinal fibers in the body wall. A roundworm 

 can bend or straighten its body but cannot twist or extend its length. 

 Segmented marine and earthworms have an outer layer of circular 

 fibers and an inner layer of longitudinal fibers in the body wall. Since 

 the body cavity is filled with fluid which is incompressible, the con- 

 traction of the circular muscles stretches the longitudinal muscles and 

 extends the body, making it longer and thinner. The contraction of the 

 longitudinal muscles makes it shorter and thicker. 



Molluscs generally have slow, nonstriated muscles, but the scallop, 

 which can swim actively by clapping its two shells together, has two 

 muscles connecting the shells. One of these is nonstriated and contracts 

 slowly, serving to keep the shells closed at rest, and the other is striated 

 and twitches rapidly to power the swimming movements. 



The arthropods have complex patterns of separate muscles rather 

 than simple layers of muscles as in the worms. These muscles vary in 

 size and attachment, and provide for the movement of the segments of 

 the body and their many-jointed appendages. The arthropod muscles 

 are located within the exoskeleton and attach to its inner surface. A 

 lobster or grasshopper has hundreds of separate muscles. 



The muscles of vertebrates are generally attached to bones or car- 

 tilages as pairs which tend to pull in opposite directions (Fig. 5.9). Since 

 muscles can pull but cannot push, this antagonistic arrangement allows 

 for movement in both directions. The end of the muscle which remains 



