Question

What is metamerism? What is the essential difference between the mode of formation of individual morphological body units of a tapeworm and those of an earthworm?

Answer 1

Segmental arrangement of the body parts in repeated sequence in an animal is called as metamerism.

Earthworm bodies are organized into a series of ring like segments. The segmentation of the mesoderm divides it internally as well. In Earthworms each segment has its own excretory, nervous, and circulatory systems and each segment work in coordination with others.  

While in tapeworms the segmentation of ectoderm is called as pseudo metamerism. There is zero coordination between segments and each unit work independently.