Sentence:  Peanut worms (phylum Sipunculida) are drab-colored bottom dwellers, most of which are less then four inches long. Their body is divided into two sections: the trunk and the <WSD>introvert</WSD>. The <WSD>introvert</WSD>, the narrow neck-like anterior section, is used as a probe. 
Meaning of the word marked with <WSD> tags:  An organ or other body part that is or can be turned inside out, especially an anterior portion of some annelid worms capable of retraction.