Question

according to the cladogram, which organisms have roundworms as a common ancestor?
○ mollusks and flatworms
○ sponges and chordates
○ flatworms, sponges, and cnidarians
○ mollusks, echinoderms, chordates, and annelids

Explanation:

To solve this, we analyze the cladogram. Roundworms (Nematodes) have a pseudocoelom. Organisms sharing a common ancestor with roundworms should be in the branch that splits after roundworms' lineage. Mollusks, annelids, echinoderms, and chordates are in the branch with coelom (after pseudocoelom - roundworms' feature). Let's check options:

• "mollusks and flatworms": Flatworms are before roundworms (no pseudocoelom), so wrong.
• "sponges and chordates": Sponges are very basal, not related via roundworms, wrong.
• "flatworms, sponges, and cnidarians": All are before roundworms' branch, wrong.
• "mollusks, echinoderms, chordates, and annelids": These are in the coelom - segmentation branch, which comes after the pseudocoelom (roundworms) branch, so they share roundworms as a common ancestor (via the ancestral line leading to pseudocoelom and then coelom).

Answer:

D. mollusks, echinoderms, chordates, and annelids (assuming the last option is D; if options are labeled as given, the correct option is the one with "mollusks, echinoderms, chordates, and annelids")