Sentence:  The anterior segments of sabellid and serpulid worms are modified in two important ways: the head has a crown of pinnate <WSD>radioles</WSD> arranged in two spirals or semicircles above the mouth, and some of the anterior segments form a thoracic region [...]. In the Serpulidae, one (sometimes two) of the <WSD>radioles</WSD> is modifed as an operculum, which blocks the mouth of the tube when the worm retreats. 
Meaning of the word marked with <WSD> tags:  A heavily ciliated feather-like tentacle occuring in clusters on the crowns of certain tubeworms, especially those of the order Canalipalpata (the (fan-head worms)), used for feeding and respiration.