Q.Describe the general features of class Polychaeta.

[!TLDR] Polychaeta are mostly marine annelids with numerous setae on parapodia, a well-developed head, and separate sexes.

[!ANSWER] Class Polychaeta consists chiefly of marine annelids bearing numerous setae grouped on paddle-like parapodia (the main locomotor structures), a well-developed head with sensory appendages, and, typically, separate sexes — well illustrated by Nereis, the ragworm.

[!ANSWER] Class Polychaeta shows a distinct set of features that separate it from the other two annelid classes.

Polychaetes are chiefly marine annelids, and each body segment typically bears a pair of fleshy, paddle-like lateral extensions called parapodia, armed with numerous chitinous setae — far more setae per segment than in Oligochaeta, and used as the class's main locomotor structures for crawling and swimming (well illustrated in Nereis, the ragworm). Polychaetes also generally possess a well-developed head region bearing sensory appendages such as tentacles, palps and, often, eyes — a much more elaborate head than the reduced head of Oligochaeta or Hirudinea. Unlike the hermaphroditic Oligochaeta and Hirudinea, the sexes in Polychaeta are usually separate (dioecious), and most polychaetes undergo indirect development via a free-swimming trochophore larva rather than direct development.

[!ANSWER] Polychaeta: mostly marine, numerous setae on locomotor parapodia (e.g. Nereis), a well-developed sensory head, separate sexes, and (typically) a trochophore larval stage — distinguishing it clearly from Oligochaeta and Hirudinea.

• Confusing polychaete parapodia with the few simple setae of Oligochaeta, or assuming Polychaeta is hermaphroditic like the other two classes.
• Forgetting to name an example (Nereis) illustrating parapodial locomotion.