Q.Distinguish class Hirudinea from class Polychaeta and class Oligochaeta.

[!TLDR] Hirudinea (leeches) differ from Polychaeta/Oligochaeta by having suckers, no/reduced setae and parapodia, a fixed segment count, and a reduced coelom. …

[!ANSWER] Hirudinea differs from Polychaeta and Oligochaeta across several structural features tied to its parasitic/predatory habit.

Polychaeta bears numerous setae on well-developed, paddle-like parapodia used for active locomotion; Oligochaeta bears only a few simple setae directly from the body wall, without parapodia; Hirudinea, by contrast, lacks parapodia entirely and has setae that are greatly reduced or completely absent. In place of setae-based locomotion, Hirudinea moves using a pair of anterior and posterior suckers, attaching and releasing them alternately in a looping motion — a locomotor and attachment strategy found in neither Polychaeta nor Oligochaeta.

The number of body segments is variable and often quite large in Polychaeta and Oligochaeta, but Hirudinea has a fixed, comparatively small number of segments. Internally, both Polychaeta and Oligochaeta retain a large, open, fluid-filled coelom that works as a hydrostatic skeleton, while in Hirudinea the coelom is greatly reduced and packed with a spongy connective tissue (botryoidal tissue), so it no longer functions as an open fluid reservoir in the same way. …

• Treating Hirudinea as simply "Oligochaeta with suckers" without noting the additional differences in coelom structure and segment-count fixity. …