Because invertebrates are so varied, there is no generalization to which an exception cannot be found. For example, lobsters and crayfish have two separate muscle fiber types in their tail musculature. The tails are important in swimming and, particularly, in escape maneuvers. They must flex and extend rapidly to evade predators or aggressors. The bulk of the tail muscles consist of short sarcomere, rapidly contracting flexors and extensors. Thin sheets of long sarcomere—slowly contracting flexors and extensors—lie near the carapace. They are used for postural adjustments and for slow movements.
Flexors and extensors are good examples of another principle of muscle action. Muscles contract and they shorten. This shortening moves a body part. Relaxation allows the muscle fibers to lengthen. The force needed to lengthen the relaxed muscle comes from the contraction of its antagonist, a muscle that produces movement in the opposite direction from that of the first muscle. For example, flexing the tail of a lobster or crayfish is performed by the tail flexors. Relaxation does not return the tail to its extended position. Contraction of the extensor muscles moves the tail away from the body and extends the tail again. Relaxation of the extensors may allow the tail to be less rigid in its extension, but the tail will not flex until the flexor muscles are contracted. Muscles work in antagonistic groups to produce opposite movements (flexion and extension) and to produce postural changes that allow a lobster or crayfish to maintain its position when the current changes direction or speed.
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Metabolism. There isn’t perhaps a more frequently used word in the weight loss (and weight gain) vocabulary than this. Indeed, it’s not uncommon to overhear people talking about their struggles or triumphs over the holiday bulge or love handles in terms of whether their metabolism is working, or not.