Q.Describe, using the six basis-of-classification criteria discussed in this chapter, how phylum Annelida differs from phylum Platyhelminthes.

Step 1. Level of organisation. Platyhelminthes has reached the organ level; Annelida has progressed further, to the organ-system level, with fully cooperating organ systems such as a closed circulatory system and segmental nephridia.

Step 2. Symmetry. Both phyla are bilaterally symmetrical — this criterion does not distinguish them.

Step 3. Diploblastic/triploblastic. Both phyla are triploblastic — this criterion also does not distinguish them.

Step 4. Coelom. This is a major point of difference: Platyhelminthes is acoelomate, with no body cavity between the gut and body wall (the space is filled with parenchyma), whereas Annelida is truly coelomate, with a body cavity completely lined by mesoderm.

Step 5. Segmentation. This is the other major difference: Platyhelminthes shows no metameric segmentation, whereas Annelida is the first phylum in the classification sequence to show true metamerism, its body divided into a linear series of repeating segments.

Step 6. Notochord. Neither phylum possesses a notochord at any stage — both remain non-chordates.

Go through all six basis-of-classification criteria one by one, stating explicitly for each whether it distinguishes the two phyla or not.

• Only discussing coelom and segmentation and skipping the criteria (symmetry, triploblasty) where the two phyla actually agree — a complete answer should state all six explicitly, not just the differing ones.
• Forgetting that neither phylum has a notochord, and mistakenly treating that criterion as a point of difference when it is in fact a shared (negative) feature.