1.

Muscles must have something rigid to pull against if they are to cause movement. In case of humans, the rigidity is supplied by ____; in the case of earthworms, the rigidity come from???.

A. A hydrostatic skeleton; an endoskeleton.
B. Opposing muscles; an exoskeleton.
C. An exoskeleton; an endoskeleton;
D. An endoskeleton; a hydrostatic skeleton.
Answer» E.


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