Question

How do the hirudineans differ from the polychaetes and oligochaetes?

Answer 1

Polychaete: mostly marine segmented worms. well developed head, each segment usually has parapapodia with chaetae, tube-dawelling and free moving. 
Oligochaeta: freshwater, marine, and terrestrial segmented worms such as earthworms, reduced head, no parapodia, chaetae present.