Which of the following is NOT an annelid?

• A. Earthworm
• B. Leech
• C. Nematode
• D. Polychaete

Answer: (C)

Annelids are defined by a segmented, metameric body plan with a true coelom, divided into compartments by septa. This segmentation is the hallmark that links earthworms, leeches, and polychaetes together. An earthworm shows clear repeating segments with bristles called setae; a leech is built from segments too, though it often appears more uniform; a polychaete is a marine annelid with many segments, often bearing parapodia.

A nematode, however, is not segmented and has a pseudocoelom instead of a true coelom. Its body is a simple, unsegmented tube with a tough outer cuticle, which is a fundamental difference from annelids. Because of this lack of segmentation and the different type of body cavity, the nematode is not an annelid.