Upon close inspection of movement in annelids and nematodes, you would notice an obvious difference. Nematodes tend to wriggle back and forth, while annelids tend to contract along their length as their girth swells and then thin out as they stretch. What anatomical feature explains this type of movement?

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Nematodes lack circular muscle fibers, which explains the difference in movement between nematodes and annelids. Annelids can contract and expand their bodies due to the presence of both longitudinal and circular muscle fibers, allowing for more complex movement patterns.

Upon Close Inspection of Movement in Annelids and Nematodes, You