While comparing invertebrate groups, a minute marine worm has a retractable head bearing spiny structures used in movement through sediment. The feature being tested is:

Kinorhynchs possess an introvert with scalids, or spiny structures, that help in anchoring and movement among marine sediments. They are small meiofaunal ecdysozoans. This MCQ belongs to the Kinorhyncha topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.